A  TREATISE 


CIVIL  EIGINEERING. 


C  BY 

D.  H.  MAHAN",  LL.D., 

LATE  PROFESSOK  OF  CIVIL  ENGINEEBINa  AT  WEST  POINT,  N.  T. 


beviseD  and  edited,  with  additions  and  new  plates, 
By  De  YOLSON  WOOD, 

PROFESSOR  OF  MATHEMATICS    AND   SIECHANXCS  IN  STEVENS'    INSTITUTE    OF  TECHNOLOOX 
(formerly  PROFESSOR   OF   CIVIL   ENGINEERING    IN   THE    UNIVERSITY  OF  MICHI- 
GAN) ;  AUTHOR    OF   A    TREATISE   ON  THE  RESISTANCE  OF  MATERIALS ; 
TREATISE  ON  BRIDGES  AND  ROOFS,  ETC. 


NEW  YORK: 
JOHN  WILEY  &  SON, 
15  astor  place. 
1873. 


Enteeed,  according  to  Act  of  Congress,  in  the  j'ear  1873,  by 
JOHN  WILEY, 
In  the  Office  of  the  Librarian  of  Congress,  at  Washington,  D.  C. 


Pooi.K  &  Maclauchlan, 

PRINTERS  AND  BOOKBINDEU?, 

205-213  Easi  xith  St., 

NEW  YORK. 


PREFACE. 


The  works  of  the  late  Professor  Mahan  are  too  well  and 
too  favorably  known  to  need  special  comment  from  the 
present  Editor. 

The  first  edition  of  his  work  on  Civil  Engineering  ap- 
peared when  engineering  as  a  learned  profession  was  scarcely 
recognized  in  this  country,  and  when  but  a  very  limited 
amount  of  instruction  upon  the  science  which  pertains  to  it 
was  given  in  our  schools.  Descriptions  of  processes  and  of 
works  executed  were  the  essential  means  of  giving  the  infor- 
mation which  was  needed  by  the  engineer.  This  determined 
the  essential  characteristic  of  his  work,  which  is  descTijjytive. 

More  recently,  numerous  schools  have  been  established, 
which  are  intended  to  give  thorough  instruction  in  the  science 
of  engineering,  and  in  which  the  courses  of  instruction  are 
largely  filled  with  mathematical  analysis.  But  analysis 
alone,  however  important,  can  never  take  the  place  of  descrip- 
tive matter.  Every  successful  structure  serves  as  a  guide  in 
the  construction  of  all  future  similar  works.  Thus  the  expe- 
rience of  one  may  become  the  wisdom  of  many. 

Before  his  untimely  death.  Professor  Mahan  had  prepared 


iv 


PREFACE. 


a  thorough  revision  of  tliis  work,  and  about  one-third  of  it 
had  passed  through  the  press  when  the  present  Editor  took 
charge  of  it. 

I  have  endeavored  to  do  full  justice  to  the  original  author 
by  preserving  the  essential  character  of  the  work,  and  retain- 
ing nearly  all  the  matter  which  he  had  prepared  ;  still,  1  have 
omitted  a  few  paragraphs  w^hich  were  deemed  non-essential, 
and  condensed  others.  I  have  also  added  considerable  new 
matter,  which  is  scattered  throughout  that  portion  of  the 
work  which  I  have  had  in  charge.  I  trust  that  my  labors 
have  added  to  the  value  of  the  work. 

De  Y.  W. 


HoBOKEN,  Aiig.,  1873, 


CONTENTS. 


CHAPTER  I. 

GENERAL  PROPERTIES  OF  BUILDING  MATERIALS. 
ABTICLE.  PAGE. 

1-2.  Introductory  Remark   3 


I.  STONE. 

3-16.  Silicioiis  Stones   4 

17-20.  Argillaceous  Stones   8 

21-29.  Calcareous  Stones   9 

30-36.  Gypsum— Durability  of  Stone   13 

37-38.  Effects  of  Heat— Hardness  of  Stone   15 


II.  LIME. 

38-41.  Classification   18 

42-49.  Hydraulic  Limes  and  Cements   19 

50-55.  Physical  Characteristics  of  Hydraulic  Limestones    23 

56-60.  Calcination  of  Limescone   25 


-III.  LIME-KILNS. 

61-76.  Descriptions  of  Lime-kilns   27 

77.  Calcination  of  the  Stone   36 

78-95.  Reducing  Calcined  Stone  to  Powder   37 

96-103.  Artificial  Hydraulic  Limes  and  Cements   41 

104-114.  Puzzolanas   43 


IV.  MORTAR. 

115-120.  Classification— Qualities   46 

121-127.  Sand,  Properties  of   47 

128-134.  Hydraulic  Mortar   48 

135-138.  Mortars  exposed  to  Weather   50 

139-142.  Manipulations  of  Mortar— Machines  for   51 

143-150.  Setting  and  Durability  of  Mortars   57 

151-152.  Theory  of  the  Hardening  of  Mortars   58 


V.   CONCRETE  BETON. 

153-157.  Concrete — Manufacture  and  Uses   59 

158-166.  Beton — Manufacture  and  Uses  .'   60 


vi  CONTENTS, 

ABTICXE.  PAOB. 

1G7.  Ransome\s  Artiticial  Stone   63 

1G8-182.  Rc'ton  Agglomcre   64 

183-180.  Adhereuce  of  Mortar   73 


VI.  .MASTICS. 

187-198.  Composition  of— Bituminous  Mastic   74 


VII.  BRICKS. 

199-211.  Common  Brick— Fire-Brick—TUe   77 


VIII.  WOOD. 

212-214.  Timber— Parts  of  the  Trunk  of  a  Tree   79 

215-217.  Felling  Trees— Time  and  Treatment   80 

218-22.1.  ^Methods  of  Seasoning  Timber   81 

226.  Wet  and  Dry  Rot  .   83 

227-239.  Preservation  of  Timber   83 

240-242.  Durability  of  Timber   86 

243-248.  Forest  Trees  of  the  United  States  which  are  used  for  Timber.  86 

IX.  METALS. 

249-203.  Cast  Iron,  Qualities  of   89 

204-277.  Wrought  Iron,  Qualities  of   92 

278-289.  Durability  of  Iron    94 

230-298.  Preservatives  of  Iron   95 

299.  Con-ugated  Iron   97 

300.  Steel   98 

301.  Copper   98 

302.  Zinc     99 

303.  Tin   99 

304.  Lead   99 


X.  PAINT  AND  VARNISHES. 

305-308.  Composition  and  Uses  of  Paints   100 

309-313.  Composition  and  Uses  of  Varnishes   100 

314.  ZoGfagous  Paint  '.103 

315.  Methods  of  Preserving  Exposed  Surfaces  of  Stone   103 


CHAPTER  II. 

EXPERIMENTAL  RESEARCHES   ON  THE  STRENGTH  OF  MATERIALS. 

310-320.  Physical  Properties  of  Bodies   106 

327-335.  Strength  of  Stone   109 

330-342.  Strength  of  Mortars  and  Cements   115 

343-347.  Strength  of  Timber   119 

34S-305.  Strength  of  Cast  Iron   131 

3f)0-307.  Effect  of  Impact  upon  Cast-Iron  Bars   144 

308-374.  Strength  of  Wrought  Iron   147 


CONTENTS.  vii 

ABTICLE.  PAGE, 

375.  Strength  of  Steel   163 

376.  Strength  of  Copper   166 

377.  Effects  of  Temperature  on  the  Tensile  Strength   167 

378.  Strength  of  Cast  Tin,  of  Cast  Lead,  hard  Gun-metal   167 

370.  Coefficients  of  Linear  Expansion  due  to  Heat   167 

380.  Adhesion  of  Iron  S^Dikes  to  Timber   170 


CHAPTER  III. 

MASONRY. 

I.  Classification  Masonry     174 

n.  Cut-stone  Masonry   174 

III.  Rubble-stone  Masonry   183 

IV.  Brick  Masonry   183 


V.   FOUNDATIONS  OF  STRUCTURES  ON  LAND. 

417-418.  Definition — Importance   190 

420.  Classification  of  Soils   190 

421-423.  Foundations  on  Rock   191 

424-425.  Foundations  on  Sand   193 

426-480.  Foundations  on  Compressible  Soils   192 

431-440.  Foundations  on  Piles   196 

441-443.  Sand  for  Bed  of  Foundation   203 


VI.   FOUNDATIONS  OF    STRUCTURES  IN  WATER. 

444-449.  Coffer- Work— Caisson   208 

452-455.  Foundations  on  Heavy  Blocks  of  Loose  Stone   217 

456.  Pneumatic  Processes   ,  .'   220 

457-458.  Pneumatic  Piles   220 

459.  Pneumatic  Caissons.    St.  Louis  Bridge   229 

East  River  Bridge   234 


VII.   CONSTRUCTION  OF  MASONRY. 

461-466.  Foundation  Courses — Construction  of   236 

467.  Classification  of  Structures  of  Masonry  -r   238 

468.  Walls  of  Enclosures   238 

469.  Walls  for  Vertical  Supports   239 

470.  Areas   239 

471-479.  Retaining  Walls.    Form  and  Dimensions   239 

480-490.  Modes  of  Strengthening  Retaining  Walls — Counterforts — Re- 
lieving Arches   244 

491-492.  Lintel— Plate-Bande   247 

493-503.  Arches — Definitions — Annular  Arches — Dome   248 

504-513.  Details  of  the  Masonry  of  Arches   252 

514-516.  Angle  of  Rupture  ■   257 

517-519.  Remarks  upon  the  Strengthening  of  Abutments   258 

520.  Precautions  against  Settling   258 

521-523.  Pointing  of  Masonry   259 

524-526.  Repairs  of  Masonry   260 

527.  Effects  of  Temperature  on  Masonry. . . . . '   261 


viii  CONTENTS. 

ARTICLE.  PAGE. 

CHAPTER  lY. 

FRAMING. 

528-539.  General  Principles   263 

540.  Solid-built  Beams   205 

541-547.  Joints  of  Dilfereut  Kinds   267 

548.  Open-built  Beams   271 

549-550.  Framing  for  Intermediate  Supports   272 

551.  Experiments  on  the  Strength  of  Frames   274 

552-553.  Wooden  Arches   277 


CHAPTEPv  V. 

BRIDGES. 

554.  Classification  of  Bridges  279 

II.    STONE  BRIDGES. 

555-556.  Location  of  Stone  Bridges    279 

557-561.  Survey — Management  of  the  Water-way  '.   280 

562.  Number  of  Bays  282 

563-567.  Classification  of  Arches— Definitions   282 

568.  Oblique  Arches  285 

569.  Arched  Bridges   289 

570-576.  Centring  for  Arches   289 

577.  Style  of  Architecture  204 

578-587  Construction  of  the  Foundations   294 

588-590.  Superstructure   302 

591-592.  Approaches — Water-wings   305 

593-594.  Enlargement  of  the  Water- w^ay — General  Remarks   306 

595-596.  Table  of  Bridget;   308 


III.  WOODEN  BRIDGES. 

597-604.  Timber  Foundations  310 

605-006.  Definitions  of  Terms  314 

607.  Long's  Truss   315 

608.  Town's  Truss  316 

609.  Howe's  Truss   317 

610.  Schuylkill  Bridge   317 

611.  Bun-'s  Truss  318 

612.  Pratt's  Truss   320 

613.  McCallum's  Truss   320 

614.  Canal  Bridge  320 

615.  Wooden  Arches   321 

G16-621.  General  Remarks   322 

622.  Architecture  of  Wooden  Bridges   324 

623.  Table  of  Wooden  Bridges  324 

IV.  CAST-IRON  BRIDGES. 

624-627.  General  Remarks   325 

628-630.  Cast-iron  Arches   327 


CONTENTS.  ix 

ABTICLH  PAGE. 

631.  Open  Cast-iron  Beams   331 

032,  Effects  of  Temperature  on  Stone  and  Cast-iron  Bridges  331 


V.   IRON  TRUSSED  BRIDGES. 

633.  Whipple's  Trapezoidal  Truss   333 

634.  Modification  of  Whipple's  Truss   334 

635.  Lim  ille's  Bridge   334 

636.  Whipple's  Arched  Truss   337 

637.  Boliman's  Truss   337 

638.  Fink's  Truss   338 

639.  Post's  Truss   339 

640.  Alleghany  River  Bridge  ,   344 

641.  St.  Louis  and  Illinois  Bridge   344 

642.  Kuilenberg  Bridge   347 


VI.   TUBULAR  BRIDGES. 

643.  Tubular  Frames  of  Wrought  Iron   347 

644.  Experiments  with  a  Model  Tube  ,  349 

645.  Britannia  Tubular  Bridge   350 

646.  Formula  for  Computing  the  Strength  of  Wrought-iron  Tubes  354 

647.  Victoria  Bridge   354 


VII.   SUSPENSION  BRIDGES, 

648-653.  General  Remarks   357 

654-655.  Anchorage   359 

650-658.  Position  and  Construction  of  the  Cables   360 

659.  Vertical  Suspension  Bars    361 

660-602,  Construction  of  Wire  Cables                                       .   361 

603-664.  Construction  of  the  Piers  and  Abutments   362 

005.  Main  Chains,  &c   303 

060.  Attachment  of  Suspending  Chains   304 

007.  Roadway   304 

008.  Vibrations   365 

669.  Means  of  Preserving  the  Chains   306 

670.  Proofs  of  Suspension  Bridges   366 

671.  Durability   367 

672.  Suspension  Bridge  near  Berwick,  England   368 

673.  Menui  Bridge   368 

674.  Fribourg  Bridge   370 

675.  Hungerford  and  Lambeth  Bridge   371 

676.  Monongahela  Wire  Bridge   373 

677.  Niagara  Railroad  Suspension  Bridge   374 

678.  East  River  Suspension  Bridge   379 


VIII.    MOVABLE  BRIDGES. 

679.  Definitions   880 

680.  Draw  Bridges   381 

681.  Turning  Bridges   384 

682.  Swing  Bridge  at  Providence,  Rhode  Island   385 

683.  Rolling  Bridges   391 

684.  Boat  Bridges   391 


X  CONTENTS. 

ABTICLE.  PAGE. 

IX.  AQUEDUCT  BRIDGES. 

685.  General  Principles   391 

C8G.  Canal  Aqueducts   392 

087.  Trunk  of  Cast  Iron  or  Timber   392 


CHAPTER  VI. 

ROOFS. 

088.  Definition   393 

689.  Kemark   393 

690.  General  Data   393 

691.  Weight  of  Snow   394 

092.  The  Force  of  Wind   394 

093.  Ordinary  Roof  Truss   394 

695.  An  Iron  Roof  Truss   396 

696.  Pvoof  of  a  Gas-House   397 

697.  An  Example  of  Trussing  ,   398 

698.  Depot  Roof  Trviss   399 

699.  Roof  of  a  Rolling  Mill   400 

700.  Truss  of  the  State  Capitol,  Vermont   400 

701.  Example  of  Roof  at  the  University  of  Michigan   402 


CHAPTER  YII. 

ROADS. 

702.  Establishing  a  Common  Road   403 

703.  Reconnaissance   403 

7047-708.  Surveying  and  Locating  Common  Roads   405 

709.  Gradients   407 

710-713.  Final  Location   408 

714-720.  Earthwork — Excavations  and  Embankments   411 

721.  Drainage   416 

722.  Road-coverings   419 

723.  Pavements — Wood  and  Stone   420 

724.  McAdara  and  Telford  Roads   423 

726.  Gravel  Roads   425 

728.  Asphaltic  Roads   427 

729.  Repairing  Common  Roads   428 

730.  Cross  Dimensions  of  Common  Roads   429 

731.  Plank  Roads   430 

II.  RAILWAYS. 

732.  Definition   430 

733-736.  Rails   430 

737.  Supports   433 

738.  Ballast   434 

739.  Temporary  Railways     434 

740.  Gauge   435 

742.  Curves   436 

743.  Sidings,  etc  ".   436 

746-747.  Gradients   438 

748-753.  Tunnels   439 


CONTENTS.  Xi 

ARTICLE.  PAGE. 

754:-768.  Experiments  of  Baron  Von  Weber  on  the  Stability  of  the 

Permanent  Way   442 

755.  Stability  of  Rails  to  Lateral  Pressure   42 

756.  Stability  of  the  Permanent  Way   -  43 

757.  Stability  of  the  Permanent  Way  to  Resist  Horizontal  Dis- 

p  acement   444 

759.  Stability  of  the  Rails  on  the  Sleepers   448 

762-763.  Experiments  on  the  Resisting-  Power  of  Railway  Spikes   451 

764.  Experiments  on  the  Effect  of  Bed-plates   454 

765.  Force  Required  to  Draw  Spikes   456 

766.  Tobal  Resistance  due  to  Spikes  and  Friction   458 

768.  Weber's  Deductions  from  Tabulated  Results   461 

769.  Sleepers   462 

770.  RailJoints   464 

771-776.  Steel  Rails   464 


CHAPTER  Yin. 

CANALS. 

777-780.  General  Remarks..;   467 

781.  Cross  Section   470 

782-783.  Supply  of  Water   471 

784-786.  Locks -Use  of,  eto   472 

787-793.  Feeders  and  Reservoirs   474 

794.  Lifts  of  Locks   479 

795.  Levels   480 

796-815.  Locks— Principles  of  Construction   482 

816-817.  Accessory  Works— Culverts   488 

818-819.  Aqueducts— Canal  Bridges   489 

820-822.  Waste -weir— Guard  Lock  ,   490 

823.  General  Dimensions  of  Canals   491 

824.  Locomotion  on  Canals   493 


CHAPTER  IX. 

RIVERS. 

825-829.  Natural  Features  of  Rivers  494 

830-831.  River  Improvements   495 

832-833.  Means  of  Protecting  the  Banks— Inundations.. . ,   496 

834-841.  Elbows— Bars   497 

842-849.  Slack- water  Navigation   501 


CHAPTER  X. 

SEA-COAST  IMPROVEMENTS. 

850-854.  Classification— Action  of  Tides   . ,  504 

855-856.  Roadsteads  506 

857-863.  Harbors   508 

864-866.  Quays  511 

867.  Dikes  512' 

868.  Groins  513 

869.  Sea-walls  -.  513: 


ELEMENTARY  COURSE 

OF 

CIYIL  ENGINEEEING. 


CHAPTER  L 


BUILDING-  MATEEIALS. 

L  Stone.  II.  Lime.  III.  Limekilns.  TV.  Mortars. 
Y.  Concretes  and  Betons.  YI.  Mastics.  YII.  Brick. 
YIII.  Wood.     IX.  Metals.     X.    Paints,  Yahnishes, 

ETC. 

summary. 


Building-Materials,  their  properties,  application,   and  classification 
(Arts.  1-2). 

L 

STONE. 

SiLicious  Stones. — Sienite,  Porphyry,  Green-Stone,  Granite  and  Gneiss, 
Mica  Slate,  Buhr  or  Mill  Stone,  Horn-Stone,  Steatite  or  Soap-Stone, 
Talcose  Slate,  and  Sand-Stone  (Arts.  3-10). 
Argillaceous  Stones. — Koofing- Slate,  Graywacke  Slate,  and  Hornblende 

Slate  (Arts.  17-20). 
Calcareous  Stones.  —Common  Limestone.    Marbles, — Statuary  Marble, 
Conglomerate  Marble,  Birdseye  Marble,    Lumachella  Marble,  Verd 
Antique  Marble,  Veined,  Golden,  Italian,  Irish,  etc. ,  Marbles.  Localities 
where  the  Limestones  and  Marbles  are  found  and  quarried  for  use  (Arts. 
21-29),  Gypsum  (Art.  30). 
Durabihty  of  Stone  (Arts.  31-36), 
Effects  of  heat  on  Stone  (Art.  37). 
Hardness  of  Stone  (Art.  38). 


2 


CTVIL  ENGINEERING. 


II. 


LIME. 

Classification  of  Lime. — Common  lime,  Hydraulic  lime,  Hydraulic 
cement,  Limestones  that  yield  Hydraulic  limes  and  Hydraulic  ce- 
ments. Analyses  of  these  stones  (Arts,  dd-^d).  Physical  characters  and 
tests  of  Hydraulic  Limestones  (Arts.  50-55).  Calcination  of  Lime- 
stones (Arts.  56-GO). 

III. 

LEMEKILNS. 

Classification  and  Kinds  of  (Arts.  61-77).  Methods  of  reducing  cal- 
cined stone  to  powder;  by  slaking- ;  by  grinding- (Arts.  78-95).  Arti- 
ficial hydraulic  limes  and  cements  (Arts.  (96-103).  Puzzolana,  etc. 
(Arts.  104-114). 

lY. 

MOKTAK. 

Classification  of  (Arts.  115-116).  Uses  of  (Art.  117).  Qualities  of, 
on  what  dependent  (Arts.  117-120).  Classification  of  Sand  (Arts.  121- 
127).  Composition  of  Hydraulic  mortar  (Arts.  128-134).  Mortar  ex- 
posed to  weather  (Arts.  135-138).  Manipydation  of  Mortar  and  Concrete 
(Arts.  139-142).  Setting  and  durabiUty  of  Mortar  (Arts.  143-150). 
Theory  of  Mortars  (151-152). 

Y. 

CONCRETES  AND  BETONS. 

Concrete  of  Common  Lime,  Manufacture  and  Uses  (Arts.  154-157). 
Beton,  its  composition,  manufacture  and  uses  (Arts.  158-161).'  Beton 
Coignet  (Arts.  162-166).  Ransome's  artificial  stone  (Art.  167).  Beton 
agglomerc  (Arts.  168-182).  Adhesion  of  Mortar  to  other  materials 
(Arts.  183-186). 

YI. 

MASTICS. 

Mastics,  Composition  of  (Art.  187).  Bituminous  Mastic,  Composition 
and  Manufacture  of  (Arts.  188-198). 

YII. 
brick. 


PRorERTiEs,  Uses  and  Manufacture  of  (Arts.  199-209).  Fire-Brick 
(Art.  210).    Tiles  (Art.  211). 


BUILDrNG  MATERIALS. 


3 


Yin. 


WOOD. 

Timber,  Kinds  op  (Art.  212).  Parts  and  properties  of  the  trunks  of 
Trees  (Arts.  213-214).  Felling  of  Trees  (Arts.  215-216).  GirdUng  and 
barking  trunks  of  Trees  (Art.  217).  Methods  of  seasoning  Timber 
(Arts.  224-225).  Wet  and  dry  rot  (Art.  220).  Preservation  of  Timber 
(Ai'ts.  227-242).    Forest  Trees  of  the  United  States  (Arts.  243-248). 


IX. 


METALS. 

Cast  Iron,  Varieties  of  (Arts.  249-263).  Wrought  Iron,  Varieties  of 
f  (Arts.  264-277).    Durability  of  Iron  (Arts.  278-289).    Preservatives  of 

Iron  (Arts.  290-298).    Corrugated  Iron  (Art.  299).    Steel  (Art.  300). 

COPPER  and  its  aUoys  (Art  301). 
ZINC  and  its  aUoys  (Art.  362). 
TIN  (Art.  303). 
LEAD  (Art.  304). 

X. 


PAINTS  AND  VARNISHES. 

Paints,  Composition,  Uses  and  Durability  of  (Arts.  305-308).  Var- 
nishes, Composition  and  Uses  of  (Arts.  309-311).  Varnish  for  Zincked 
Iron  (Arts.  312-313).  Zoofagous  Paint  (Art  314).  Methods  of  preserv- 
ing exposed  surfaces  of  Stone  (Art.  315). 

1.  A  KNOWLEDGE  of  tliG  properties  of  building  materials 
is  one  of  the  most  important  branches  of  Civil  Engineering. 
An  engineer,  to  be  enabled  to  make  a  judicious  selection 
of  materials,  and  to  apply  them  so  that  the  ends  of  sound 
economy  and  skilful  workmanship  shall  be  equally  sub- 
served, must  know : — 

1st.  Their  ordinary  durability  imder  the  various  circum- 
stances in  which  they  are  employed,  and  the  means  .of  in- 
creasing it  when  desirable. 

2d.  Their  capacity  to  sustain,  without  injury  to  their 
physical  qualities,  permanent  strains,  whether  exerted  to 
crush  them,  tear  them  asunder,  or  to  break  them  trans- 
versely. 


4 


CIVIL  ENGINEERING. 


3d.  Their  resistance  to  rupture  and  wear,  from  percussion 
and  attrition. 

4th.  Finally,  the  time  and  expense  necessary  to  convert 
them  to  the  uses  for  which  they  may  be  required. 

2.  The  materials  in  general  use  for  civil  constructions 
may  be  arranged  under  the  three  following  heads : — 

1st.  Those  which  constitute  the  more  solid  components  of 
structures,  as  Stone,  Brick,  Wood,  and  the  Metals. 

2d.  The  cements  in  general,  as  Mortar,  Mastics,  Glue, 
etc.,  which  are  used  to  unite  the  more  solid  parts. 

3d.  The  various  mixtures  and  chemical  preparations,  as 
solutions  of  Salts,  Paints,  Bituminous  Substances,  etc., 
employed  to  coat  the  more  solid  parts,  and  protect  them 
from  the  chemical  and  mechanical  action  of  atmospheric 
changes,  and  other  causes  of  destructibility. 


1. 

STONE. 

3.  The  term  Stone,  or  Roch,  is  applied  to  any  aggregation 
of  several  mineral  substances. . 

Stones,  for  the  convenience  of  description,  may  be  arranged 
under  three  general  heads — ^the  silicious,  the  argillaceous, 
and  the  calcareous, 

4.  SILICIOUS  STONES.  The  stones  arranged  under 
this  head  receive  their  appellation  from  silex,  the  principal 
constituent  of  the  minerals  which  compose  them.  They  are 
also  frequently  designated,  either  according  to  the  mineral 
found  most  al)undantly  in  them,  or  from  the  appearance  of 
the  stone,  ii^  feldspathic,  quartzose,  arenaceous,  etc. 

5.  The  silicious  stones  generally  do  not  effervesce  with 
acids,  and  emit  sparks  when  struck  with  a  steel.  They  pos- 
sess, in  a  high  degree,  the  properties  of  strength,  hardness, 
and  durability ;  and,  although  presenting  great  diversity  in 
the  degree  of  these  properties,  as  well  as  in  their  structure, 
they  furnish  an  extensive  variety  of  the  best  stone  for  the 
various  purposes  of  the  engineer  and  architect. 

6.  Sienite,  Po7^hyry,  and  Green-stone,  from  the  abmi- 


BUILDING  MATERIALS. 


5 


dance  of  feldspar  which  they  contain,  are  often  designated 
as  feldspathic  rocks.  For  durability,  strength,  and  hard- 
ness, they  may  be  placed  in  the  first  rank  of  silicions 
stones. 

7.  Sienite  consists  of  a  granular  aggregation  of  feldspar, 
hornblende,  and  quartz.  It  furnishes  one  of  the  most  valua- 
ble building  stones,  particularly  for  structures  which  require 
great  strength,  or  are  exposed  to  any  very  active  causes  of 
destructibility,  as  sea  walls,  lighthouses,  and  fortifications. 
Sienite  occurs  in  extensive  beds,  and  may  be  obtained,  from 
the  localities  where  it  is  quarried,  in  blocks  of  any  requisite 
size.  It  does  not  yield  easily  to  the  chisel,  owing  to  its  great 
hardness,  and  when  coarse-grained  it  cannot  be  wrought  to  a 
smooth  surface.  Like  all  stones  in  which  feldspar  is  found, 
the  durability  of  sienite  depends  essentially  upon  the  com- 
position of  this  mineral,  which,  owing  to  the  potash  it  con- 
tains, sometimes  decomposes  very  rapidly  when  exposed  to 
the  weather.  The  durability  of  feldspathic  rocks,  however, 
is  very  variable,  even  where  their  composition  is  the  same ; 
no  pains  should  therefore  be  spared  to  ascertain  this  prop- 
erty in  stone  taken  from  new  quarries,  before  using  it  for 
important  public  works. 

8.  Porphyry.  This  stone  is  usually  composed  of  com- 
pact feldspar,  having  crystals  of  the  same,  and  sometimes 
those  of  other  minerals,  scattered  through  the  mass.  Por- 
phyry furnishes  stones  of  various  colors  and  texture ;  the 
usual  color  being  reddish,  approaching  to  purple,  from  which 
the  stone  takes  its  name.  One  of  the  most  beautiful  varie- 
ties is  a  hrecciated  porphyry,  consisting  of  angular  fragments 
of  the  stone  united  by  a  cement  of  compact  feldspar. 
Porphyry,  from  its  rareness  and  extreme  hardness,  is  seldom 
applied  to  any  other  than  ornamental  purposes.  The  best 
known  localities  of  sienite  and  porphyry  in  the  United 
States  are  in  the  neighborhood  of  Boston. 

9.  Green-stone.  This  stone  is  a  mixture  of  hornblende 
with  common  and  compact  feldspar,  presenting  sometimes  a 
granular  though  usually  a  compact  texture.  Its  ordinary 
color,  when  dry,  is  some  shade  of  brown ;  but,  when  wet,  it 
becomes  greenish,  from  which  it  takes  its  name.  Green- 
stone is  very  hard,  and  one  of  the  most  durable  rocks ;  but, 
occurring  in  small  and  irregular  blocks,  its  uses  as  a  build- 
ing stone  are  very  restricted.  When  walls  of  this  stone  are 
built  with  very  white  mortar,  they  present  a  picturesque  ap- 
pearance, and  it  is  on  that  account  well  adapted  to  rural 
architecture.    Green-stone  might  also  be  used  as  a  material 


6 


CIYIL  ENGINEERING. 


for  road-making;  large  quantities  of  it  are  annually  taken 
from  the  principal  locality  of  this  rock  in  the  United 
States,  so  well  known  as  the  Palisades,  on  tlie  Hudson,  for 
constructing  wharves,  as  it  is  found  to  withstand  well  the 
action  of  salt  water. 

10.  Granite  and  Gneiss.  The  constituents  of  these  two 
stones  are  the  same,  being  a  granular  aggregation  of  quartz, 
feldspar,  and  mica,  in  variable  proportions.  They  differ  only 
iji  their  structure — gneiss  being  a  stratified  rock,  the  ingre- 
dients of  which  occur  frequently  in  a  more  or  less  laminated 
state.  Gneiss,  although  less  valuable  than  granite,  owing  to 
the  effect  of  its  structure  on  the  size  of  the  blocks  which  it 
yields,  and  from  its  not  splitting  as  smoothly  as  granite 
across  its  beds  of  stratification,  furnishes  a  building  stone 
suitable  for  most  architectural  purposes.  It  is  also  a  good 
flagging  material,  when  it  can  be  obtained  in  thin  slabs. 

Granite  varies  greatly  in  quality  according  to  its  texture 
and  the  relative  proportion  of  its  constituents.  Wlien  the 
quartz  is  in  excess,  it  renders  the  stone  hard  and  brittle,  and 
very  difficult  to  be  worked  with  the  chisel.  An  excess  of 
mica  usually  makes  the  stone  friable.  An  excess  of  feldspar 
gives  the  stone  a  white  hue,  and  makes  it  freer  under  the 
chisel.  The  best  granites  are  those  w^ith  a  fine  grain,  in 
which  the  constituents  seem  uniformly  disseminated  through 
the  mass.  The  color  of  granite  is  usually  some  shade  of 
gray  ;  when  it  varies  from  this,  it  is  owing  to  the  color  of  the 
feldspar.  One  of  its  varieties,  known  as  Oriental  granite, 
has  a  fine  reddish  hue,  and  is  chiefly  used  for  ornamental 
purposes.  Granite  is  sometimes  mistaken  for  sienite,  wdien 
it  contains  but  little  mica. 

The  quality  of  granite  is  affected  by  the  foreign  minerals 
which  it  may  contain  ;  hornblende  is  said  to  render  it  tough, 
and  schorl  makes  it  quite  brittle.  The  protoxide  and  sul- 
phurets  of  iron  are  the  most  injurious  in  their  effects  on 
granite  ;  the  former  by  conversion  into  a  peroxide,  and  the 
latter,  l)y  decomposing,  destroying  the  structure  of  the  stone, 
and  causing  it  to  break  up  and  disintegrate. 

Granite,  gneiss,  and  sienite,  differ  so  little  in  their  essen- 
tial qualities,  as  a  building  material,  that  they  may  be  used 
indifferently  for  all  structui-es  of  a  solid  and  durable  charac- 
ter. They  are  extensively  quarried  in  most  of  the  New 
England  States,  in  New  York,  and  in  some  of  the  other 
States  intersected  by  the  great  range  of  primitive  rocks, 
where  the  quarries  lie  contiguous  to  tidewater. 

11.  Mica  Slate.    The  constituents  of  this  stone  are  quartz 


BUILDINa  MATERIALS. 


aiid  mica,  the  latter  predominating.  It  is  principally  used 
as  a  flagging  stone,  and  as  a  fire  stone,  or  lining  for  fur- 
naces. 

12.  Buhr  or  Mill  stolie.  This  is  a  very  hard,  durable 
stone,  presenting  a  peculiar,  honeycomb  appearance.  It 
makes  a  good  building  material  for  common  purposes,  and 
is  also  suitable  for  road  coverings. 

13.  Horn-stone.  This  is  a  highly  silicious  and  very  hard 
stone.  It  resembles  flint  in  its  structure,  and  takes  its  name 
from  its  translucent,  horn-like  appearance.  It  furnishes  a 
very  good  road  material. 

14. '  Steatite,  or  Soap-stone.  This  stone  is  a  partially 
indurated  talc.  It  is  a  very  soft  stone,  not  suitable  for  ordi- 
nary building  purposes.  It  furnishes  a  good  fire-stone,  and 
is  used  for  the  lining  of  fireplaces. 

15.  Taloose  Slate.  This  stone  resembles  mica  slate,  be- 
ing an  aggregation  of  quartz  and  talc.  It  is  applied  to  the 
same  purposes  as  mica  slate. 

16.  Sand-stone.  This  stone  consists  of  grains  of  silicious 
sand,  arising  from  the  disintegration  of  silicious  rocks, 
which  are  united  by  some  natural  cement,  generally  of  an 
ar2;illaceous  or  a  silicious  character. 

The  strength,  liardness,  and  durability  of  sand-stone  vary 
between  very  wide  limits.  Some  varieties  being  little  in- 
ferior to  good  granite,  as  a  building  stone,  others  being  very 
soft,  friable,  and  disintegrating  rapidly  when  exposed  to  the 
weather.  The  least  durable  sand-stones  are  those  which  con- 
tain the  most  argillaceous  matter  ;  those  of  a  f  eldspathic  char- 
acter are  also  found  not  to  withstand  well  the  action  of  the 
weather. 

Sand-stone  is  used  very  extensively  as  a  building  stone,  for 
flagging,  for  road  materials,  and  some  of  its  varieties  furnish 
an  excellent  fire-stone.  Most  of  the  varieties  of  sand-stone 
yield  readily  under  the  chisel  and  saw,  and  split  evenly,  and, 
from  these  properties,  have  received  from  workmen  the  name 
oi  freestone.  The  colors  of  sand-stone  present  also  a  variety 
of  shades,  principally  of  gray,  brown,  and  red. 

The  formations  of  sand-stone  in  the  United  States  are  very 
extensive,  and  a  number  of  quarries  are  worked  in  JS'ew 
England,  New  York,  and  the  Middle  States.  These  forma- 
tions, and  the  character  of  the  stone  obtained  from  them,  are 
minutely  described  in  the  Geological  Reports  of  these 
States,  which  have  been  published  within  the  last  few 
yeai*s. 

Most  of  the  stone  used  for  the  public  buildings  in  Wash- 


8 


CIYIL  ENGINEEEING. 


ington  is  a  sand-stone  obtained  from  quarries  on  Acquia 
Creek  and  the  Rappahannock.  Much  of  this  stone  is  felds- 
pathic,  possesses  but  little  strength,  and  disintegrates  rapidly. 
The  red  sand-stones  which  are  used'  in  our  large  cities  are 
either  from  quarries  in  a  formation  extending  from  the 
Hudson  to  North  Carolina,  or  from  a  separate  deposit  in  the 
Yalley  of  the  Connecticut.  The  most  durable  and  hard 
portions  of  these  formations  occur  in  the  neighborhood  of 
trap  dikes.  The  line  flagging-stone  used  in  our  cities  is 
mostly  obtained  either  from  the  Connecticut  quarries,  or 
from  others  near  the  Hudson,  in  the  Catskill  group  of 
mountains.  Many  quarries,  which  yield  an  excellent  build- 
ing stone,  are  woi'ked  in  the  extensive  formations  along  the 
Appalachian  range,  which  extends  through  the  interior, 
through  New  York  and  Yirginia,  and  the  intermediate 
States. 

17.  Argillaceous  Stones.  The  stones  arranged  under 
this  head  are  mostly  composed  of  clay,  in  a  more  or  less 
indurated  state,  and  presenting  a  laminated  structure.  They 
vary  greatly  in  strength,  and  are  generally  not  durable, 
decomposing  in  some  cases  very  rapidly,  from  changes  in 
the  metallic  sulphurets  and  salts  found  in  most  of  them. 
The  uses  of  this  class  of  stones  are  restricted  to  roofing  and 
flao:e:in2:. 

18.  Roofing  Slate.  This  well-kno^vn  stone  is  obtained 
from  a  hard,  indurated  clay,  the  surfaces  of  the  lamina 
having  a  natural  polish.  The  best  kinds  split  into  thin, 
uniform,  light  slabs ;  are  free  from  sulphurets  of  iron ; 
give  a  clear  ringing  sound  when  struck ;  and  absorb  but 
little  water.  Much  of  the  roofing  slate  quarried  in  the 
United  States  is  •  of  a  very  inferior  quality,  and  becomes 
rotten,  or  decomposes,  after  a  few  years'  exposure.  The 
durability  of  the  best  European  slate  is  about  one  hundred 
years ;  and  it  is  stated  that  the  material  obtained  from  some 
of  the  quarries  worked  in  the  United  States  is  not  apparently 
inferior  to  the  best  foreign  slate  brought  into  our  markets. 
Several  quarries  of  roofing  slate  are  worked  in  the  New 
England  States,  New  York  and  Pennsylvania. 

19.  Graywacke  Slate.  The  composition  of  this  stone 
is  mostly  indurated  clay.  It  has  a  more  earthy  appearance 
than  argillaceous  slate,  and  is  generally  distinctly  arenace- 
ous. Its  colors  are  usually  dark  gray,  or  red.  It  is  quarried 
principally  for  flagging-stone. 

20.  Hornblende  Slate.  This  stone,  known  also  as  green- 
stone slate,  properly  belongs  to  the  silicious  class.    It  con- 


BTJILDING  MATEETAL8. 


9 


gists  mostly  of  liornblende  having  a  laminated  structure.  It 
is  chiefly  quarried  for  liaggino^-stone. 

21.  Calcareous  Stones.  Lime  is  the  principal  constitu- 
ent of  tliis  class,  the  carbonates  of  which,  known  as  lime- 
stone and  marble^  furnish  a  large  amount  of  ordinary  build- 
ing stone,  most  of  the  ornamental  stones,  and  the  chief  in- 
gredient in  the  composition  of  the  cements  and  mortars  used 
in  stone  and  brick  work.  Limestone  effervesces  copiously 
with  acids  ;  its  texture  is  destroyed  by  a  strong  heat,  which 
also  drives  off  its  carbonic  acid  and  water,  converting  it  into 
quich  lime.  By  absorbing  water,  quick-lime  is  converted  into 
a  hydrate^  or  what  is  known  as  slaked  lime ;  considerable 
heat  is  evolved  during  this  chemical  change,  and  the  stone 
increases  in  bulk,  and  gradually  crumbles  down  into  a  fine 
powder. 

The  limestones  present  great  diversity  in  their  physical 
properties.  Some  of  them  seem  as  durable  as  the  best  sili- 
cious  stones,  and  are  but  little  inferior  to  them  in  strength 
and  hardness ;  others  decompose  rapidly  on  exposure  to  the 
weather;  and  some  kinds  are  so  soft, that  when  first  quarried, 
they  can  be  scratched  with  the  nail,  and  broken  between  the 
fingers. 

The  limestones  are  generally  impure  carbonates ;  and 
we  are  indebted  to  these  impurities  for  some  of  the 
most  beautiful,  as  well  as  the  most  valuable  materials  used 
for  constructions.  Those  which  are  colored  by  metallic 
oxides,  or  by  the  presence  of  other  minerals,  furnish  the 
large  number  of  colored  and  variegated  marbles ;  while  those 
which  contain  a  certain  proportion  of  clay,  or  of  magnesia, 
yield,  on  calcination,  those  cements  which,  from  their  posses- 
sing the  property  of  hardeiiing  under  water,  Jiave  received  the 
various  appellations  of  liydrauliG  lime,  water  lime,  Roman 
cement,  etc. 

Limestone  is  divided  into  two  principal  classes,  granular 
limestone  and  compact  limestone.  Each  of  these  furnishes 
both  the  marbles  and  ordinary  building  stone.  The  varieties 
not  susceptible  of  receiving  a  polish  are  sometimes  called 
common  limestone. 

The  granular  limestones  are  generally  superior  to  the 
compact  for  building  purposes.  Those  which  have  the 
finest  grain  are  the  best,  both  for  marbles  and  ordinary 
building  stone.  The  coarse-grained  varieties  are  frequently 
friable,  and  disintegrate  rapidly  when  exposed  to  the  weather. 
All  the  varieties,  both  of  the  compact  and  granular,  work 
freely  under  the  chisel  and  grit-saw,  and  may  be  obtained 


10 


OrVTL  ENGINEERING. 


in  blocks  of  any  suitable  dimensions  for  the  heaviest  struc- 
tures. 

The  durability  of  limestone  is  very  materially  affected 
by  the  foreip^n  minerals  it  may  contain ;  the  presence  of 
clay  injures  the  stone,  particularly  when,  as  sometimes  hap- 
pens, it  runs  through  the  bed  in  very  minute  veins  :  blocks 
of  stone  having  this  imperfection  soon  separate  along  these 
veins  on  exposure  to  moisture.  The  protoxide,  the  proto-car- 
bonate,  and  the  sulphuret  of  iron,  are  also  very  destructive  in 
their  effects ;  frequently  causing,  by  their  chemical  changes, 
rapid  disintegration. 

Among  the  varieties  of  impure  carbonates  of  lime,  the 
magnesian  limestones,  called  dolomites,,  merit  to  be  particu- 
larly noticed.  They  are  regarded  in  Europe  as  a  superior 
building  material ;  those  being  considered  the  best  which 
are  most  crystalline,  and  are  composed  of  nearly  equal  pro- 
portions of  the  carbonates  of  lime  and  magnesia.  Some  of 
the  quarries  of  this  stone,  which  have  been  opened  in  IS^ew 
York  and  Massachusetts,  have  given  a  different  result ;  the 
stone  obtained  from  them  being,  in  some  cases,  extremely 
friable. 

22.  Marbles. — The  term  marble  is  now  applied  exclu- 
sively to  any  limestones  which  will  receive  a  polish.  Owing 
to  the  cost  of  preparing  marble,  it  is  mostly  restricted  in  its 
uses  to  ornamental  purposes.  The  marbles  present  great 
variety,  both  in  color  and  appearance,  and  have  generally 
received  some  appropriate  name  descriptive  of  these  acci- 
dents. 

23.  Statuary  Marble  is  of  the  purest  white,  finest  grain, 
and  free  from  all  foreign  minerals.  It  receives  that  delicate 
polish,  without  glare,  which  admirably  adapts  it  to  the  pur- 
poses of  the  sculptor,  for  whose  use  it  is  mostly  reserved. 

24.  Conglomerate  Marble.  This  consists  of  two  varie- 
ties ;  the  one  termed  pudding  stone,  which  is  composed  of 
rounded  pebbles  embedded  in  compact  limestone ;  the  other 
termed  h^eccia,  consisting  of  angular  fragments  united  in  a 
similar  manner.  The  colors  of  these  marbles  are  generally 
variegated,  forming  a  very  handsome  ornamental  material. 

25.  Bird's-eye  Marble.  The  name  of  this  stone  is  de- 
scriptive of  its  appearance,  which  arises  from  the  cross  sec- 
tions of  a  peculiar  fossil  {fucoides  demissus)  contained  in 
the  mass,  made  in  sawing  or  splitting  it. 

26.  Lumachella  Marble.  This  is  obtained  from  a  lime- 
stone having  shells  embedded  in  it,  and  takes  its  name  from 
this  circumstance. 


BUILDING  MATERIALS. 


11 


27.  Verd  Antique.  This  is  a  rare  and  costly  variety,  of 
a  beautiful  green  color,  caused  by  veins  and  blotches  of  ser- 
pentine diffused  through  the  limestone. 

28.  The  terms  veined,  golden,  Italian,  Irish,  etc.,  given  to 
the  marbles  found  in  our  markets  are  significant  of  their  ap- 
pearance, or  of  the  localities  from  which  they  are  procured. 

29.  Limestone  is  so  extensively  diffused  throughout  the 
United  States,  and  quarried,  either  for  building  stone  or 
to  furnish  lime,  in  so  many  localities,  that  it  would  be  im- 
practicable to  enumerate  all  within  any  moderate  compass. 
One  of  the  most  remarkable  formations  of  this  stone  extends, 
in  an  uninterrupted  bed,  from  Canada,  through  the  States  of 
Vermont,  Mass.,  Conn.,  New  York,  STew  Jersey,  Penn.,  and 
Virg.,  and  in  all  j^robability  much  farther  south. 

Marbles  are  quarried  in  various  localities  in  the  United 
States.  Among  the  most  noted  are  the  quarries  in  Berk- 
shire Co.,  Mass.,  which  furnish  both  pure  and  variegated 
marbles  ;  those  on  the  Potomac,  from  which  the  columns  of 
conglomerate  marbles  were  obtained  that  are  seen  in  the 
interior  of  the  Capitol  at  Washington;  several  in  IS'ewYork, 
which  furnish  white,  the  bird's-eye,  and  other  variegated 
kinds;  and  some  in  Conn.,  which,  among  other  varieties, 
furnish  a  verd  antique  of  handsome  quality. 

Limestone  is  burned,  either  for  building  or  agricultural 
purposes,  in  almost  every  locality  where  deposits  of  tlie 
stone  occur.  Thomaston,  in  Maine,  has  supplied  for  some 
years  most  of  the  markets  on  the  sea-board  with  a  material 
which  is  considered  as  a  superior  article  for  ordinary  build- 
ing purposes.  One  of  the  greatest  additions  to  the  building 
resources  of  our  country  was  made  in  the  discovery  of  the 
hydraulic  or  water  limestones  of  New  York.  The  prepara- 
tion of  this  material,  so  indispensable  for  all  hydraulic  works 
and  heavy  structures  of  stone,  is  carried  on  extensively  at 
Pondout,  on  the  Delaware  and  Hudson  canal,  in  Madison  Co., 
and  is  sent  to  every  part  of  the  United  States,  being  in 
great  demand  for  all  the  public  works  carried  on  under  the 
superintendence  of  our  civil  and  military  engineers.  A  not 
less  valuable  addition  to  our  building  materials  has  been 
made  by  Prof.  W.  B.  Rogers,  who,  a  few  years  since,  direct- 
ed the  attention  of  engineers  to  the  dolomites,  for  their  good 
hydraulic  properties.  From  experiments  made  by  Vicat, 
in  France,  who  first  there  observed  the  same  properties  in 
the  dolomite,  and  from  those  in  our  country,  it  appears  highly 
probable  that  the  magnesian  limestones,  containing  a  cer- 
tain proportion  of  magnesia,  will  be  found  fully  equal  to 


12 


CIVIL  ENGINEERING. 


the  argillaceous,  from  which  hydraulic  lime  has  hitherto 
been  solely  obtained. 

Both  of  these  limestones  belong  to  very  extensive  forma- 
tions. The  hydraulic  limestones  of  New  York  occur  in  a 
deposit  called  the  Water-lime  Group,  in  the  Geological  Survey 
of  New  York  corresponding  to  formation  YI.  of  Prof.  H. 
B.  Rogers'  arrangement  of  the  rocks  of  Penn.  This  forma- 
tion is  co-extensive  with  the  Ilelderberg  Pange  as  it  crosses 
New  York ;  it  is  exposed  in  many  of  the  valleys  of  Penn. 
and  Yir.,  west  of  the  Great  Yalley.  It  may  be  sought  for 
just  below  or  not  far  beneath  the  Oriskany  sand-stones  of 
the  New  York  Survey,  which  correspond  to  formation  YII. 
of  Pogers.  This  sand-stone  is  easily  recognized,  being  of  a 
yellowish  white  color,  granular  texture,  with  large  cavities 
left  by  decayed  shells.  The  limestone  is  usually  an  earthy 
drab-colored  rock,  sometimes  a  greenish  blue,  which  does  not 
slake  after  being  burned. 

The  hydraulic  magnesian  limestones  belong  to  the  for- 
mations ll.  and  YI.  oi  Pogers ;  the  first  of  these  is  the  same 
as  the  Black  Piver  or  Mohawk  limestone  of  the  New  York 
Survey.  It  is  the  oldest  fossiliferous  limestone  in  the  United 
States,  and  occurs  throughout  the  whole  bed,  associated  with 
the  slates  which  occupy  formation  III.  of  Pogers,  and  are 
called  the  Hudson  Piver  Group  in  the  New  York  Survey. 
This  extensive  bed  lies  in  the  great  Appalachian  Yalley, 
known  as  the  Yalley  of  Lake  Champlain,  Yalley  of  the  Hud- 
son, as  far  as  the  Highlands,  Cumberland  Yalley,  Yalley  of 
Yirginia,  and  Yalley  of  East  Tennessee.  The  same  stone  is 
found  in  the  deposits  of  some  of  the  western  valleys  of  the 
mountain  region  of  Penn.  and  Yirginia. 

Thus  far  no  deposits  of  hydraulic  limestones  have  been 
found  on  the  Pacihc  Coast. 

The  importance  of  hydraulic  lime  to  the  security  of  struc- 
tures exposed  to  constant  moisture  renders  a  knowledge  of 
the  geological  positions  of  those  limestones  from  which  it 
can  be  obtained  an  object  of  great  interest.  From  the  results 
of  the  various  geological  surveys  made  in  the  United  States 
and  in  Europe,  limestone,  possessing  hydraulic  properties 
when  calcined,  may  be  looked  for  among  those  beds  which 
are  found  in  connection  with  the  shales,  or  other  argillaceous 
deposits.  The  celebrated  Roman  or  Parker'^s  cement,  of 
England,  which,  from  its  prompt  induration  in  water,  has 
become  aii  important  article  of  commerce,  is  manufactured 
fi-om  nodules  of  a  concretionary  argillaceous  limestone,  called 
septaria^  from  being  traversed  by  veins  of  sparry  carbonate 


BUILDINa  MATERIALS. 


13 


of  lime.  Modules  of  this  character  are  found  in  Mass.,  and 
in  some  other  States ;  and  it  is  probable  they  would  yield,  if 
suitably  calcined  and  ground,  an  article  in  nowise  inferior  to 
that  imported. 

30.  GYPSUM,  or  PLASTER  of  PARIS.  This  stone  is 
a  sulphate  of  lime,  and  has  received  its  name  from  the  exten- 
sive use  made  of  it  at  Paris,  and  in  its  neighborhood,  where 
it  is  quarried  and  sant  to  all  parts  of  the  world ;  being  of  a 
superior  quality,  owing,  it  is  stated,  to  a  certain  portion  of 
carbonate  of  lime  which  the  stone  contains.  Gypsum  is  a 
very  soft  stone,  and  is  not  used  as  a  bailding  stone.  Its  chief 
utility  is  in  furnishing  a  beautiful  material  for  the  ornamental 
casts  and  mouldings  in  the  interior  of  edifices.  For  this  pur- 
pose it  is  prepared  by  calcining,  or,  as  the  workmen  term  it, 
boiling  the  stone,  until  it  is  deprived  of  its  water  of  crystal- 
lization. In  this  state  it  is  made  into  a  thin  paste,  and  poured 
into  moulds  to  form  the  cast,  in  which  it  hardens  very 
promptly.  Calcined  plaster  of  Paris  is  also  used  as  a  cement 
for  stone ;  but  it  is  eminently  unfit  for  this  purpose ;  for 
when  exposed,  in  any  situation,  to  moisture,  it  absorbs  it  with 
avidity,  swells,  cracks,  and  exfoliates  rapidly. 

Gypsum  is  found  in  various  localities  in  the  United  States. 
Large  quantities  of  it  are  quarried  in  New  York,  both  for 
building  and  agricultural  purposes. 

31.  DURABILITY  OF  STONE.  The  most  important 
properties  of  stone,  as  a  building  material,  are  its  durability 
under  the  ordinary  circumstances  of  exposure  to  weather  ; 
its  capacity  to  sustain,  without  change,  high  degrees  of  tem- 
perature ;  and  its  resistance  to  the  destructive  action  of  fresh 
and  salt  water. 

The  wear  of  stone  from  ordinary  exposure  is  very  variable, 
depending,  not  only  upon  the  texture  and  constituent  elements 
of  the  stone,  but  also  upon  the  locality  and  position  it  may  oc- 
cupy in  a  structure,  with  respect  to  the  prevailing  driving 
rains.  The  chemist  and  geologist  have  not,  thus  far,  laid  down 
any  infallible  rules  to  guide  the  engineer  in  the  selection  of  a 
material  that  may  be  pronounced  durable  for  the  ordinary 
period  allotted  to  the  works  of  man.  In  truth  the  subject  ad- 
mits of  only  general  indications  ;  for  stones  having  tlie  same 
texture  and  chemical  composition,  from  causes  not  fully  as- 
certained, are  found  to  possess  very  different  degrees  of  dura- 
tion. This  has  been  particularly  noted  in  feldspathic  rocks. 
As  a  general  rule,  those  stones  which  are  fine-grained,  absorb 


14 


CIVIL  ENGINEERmQ. 


least  water,  and  are  of  greatest  specific  gravity,  are  also  most 
durable  under  ordinary  exposures.  The  weight  of  a  stone, 
however,  may  arise  from  a  large  proportion  of  iron  in  the  state 
of  a  protoxide,  a  circumstance  generally  unfavorable  to  its 
durability.  Besides  the  various  chemical  combinations  of  iron, 
potash  and  clay,  when  found  in  considerable  quantities,  both 
in  the  primary  and  sedimentary  silicious  rocks,  greatly  affect 
their  durability.  The  potash  contained  in  feldspar  dissolves, 
and,  carrying  off  a  considerable  proportion  of  the  silica,  leaves 
nothing  but  aluminous  matter  behind.  The  clay,  on  the  other 
hand,  absorbs  water,  becomes  soft,  and  causes  the  stone  to 
crumble  to  pieces.  Iron  in  the  form  of  protoxide,  in  some  cases 
only,  discolors  the  stone  by  its  conversion  into  a  peroxide. — 
This  discoloration,  while  it  greatly  diminishes  the  value  of 
some  stones,  as  in  white  marble,  in  others  is  not  disagreeable 
to  the  eye,  producing  often  a  mottled  appearance  in  buildings 
which  adds  to  the  picturesque  effect. 

32.  Frost,  or  rather  the  alternate  actions  of  freezing  and 
thawing,  is  the  most  destructive  agent  of  I^ature  with  which 
the  engineer  has  to  contend.  Its  effects  vary  with  the  texture 
of  stones  ;  those  of  a  fissile  nature  usually-  splitting,  while  the 
more  porous  kinds  disintegrate,  or  exfoliate  at  the  surface. — 
When  stone  from  a  new  quarry  is  to  be  tried,  the  best  indication 
of  its  resistance  to  frost  may  be  obtained  from  an  examination 
of  any  rocks  of  the  same  kind,  within  its  vicinity,  which  are 
known  to  have  been  exposed  for  a  long  period.  Submitting 
the  stone  fresh  from  the  quarry  to  the  direct  action  of  freez- 
ing would  seem  to  be  the  most  certain  test,  were  the  stone 
destroyed  by  the  expansive  action  of  the  frost  ;  but 
besides  the  uncertainty  of  this  test,  it  is  known  that  some 
stones,  which,  when  first  quarried,  are  much  affected  by  frost, 
splitting  under  its  action,  become  impervious  to  it  after  they 
have  lost  the  moisture  of  the  quarry,  as  they  do  not  re- absorb 
near  so  large  an  amount  as  they  bring  from  the  quarry. 

33.  M.  Brard,  a  French  chemist,  has  given  a  process  for 
ascertaining  the  effects  of  frost  on  stone,  which  has  met  with 
the  approval  of  many  French  architects  and  engineers  of 
standing,  as  it  corresponds  with  their  experience.  M.  Brard 
directs  that  a  small  cubical  block,  about  two  inches  on  the 
edge,  shall  be  carefully  sawed  from  the  stone  to  be  tested.  A 
cold  saturated  solution  of  sulphate  of  soda  is  prepared,  placed 
over  a  fire,  and  brought  to  the  boiling  point.  The  stone,  sus- 
pended from  a  string,  is  immersed  in  the  boiling  liquid,  and 
kept  there  during  thirty  minutes ;  it  is  then  carefully  with- 
drawn ;  the  liquid  is  decanted  free  from  sediment  into  a  fiat 


BUILDING  MATERIALS. 


15 


vessel,  and  the  stone  is  suspended  over  it  in  a  cool  cellar.  An 
efflorescence  of  the  salt  soon  makes  its  appearance  on  the 
stone,  when  it  must  be  again  dipped  into  the  liquid.  This 
should  be  done  once  or  more  frequently  during  the  day,  and 
the  process  be  continued  in  this  way  for  about  a  week.  The 
earthy  sediment,  found  at  the  end  of  this  period  in  the  vessel, 
is  weighed,  and  its  quantity  will  give  an  indication  of  the  like 
effect  of  frost.  This  process,  with  the  official  statement  of  a 
commission  of  engineers  and  architects,  by  whom  it  was  test- 
ed, is  minutely  detailed  in  vol.  38,  Annales  de  Chimie  et  de 
Physique,  and  the  results  are  such  as  to  commend  it  to  the 
attention  of  engineers  in  submitting  new  stones  to  trial. 

34.  From  more  recent  experiments  by  Dr.  Owen  it  was 
found  that  the  results  obtained  by  exposing  the  more  porous 
stones  to  the  alternate  action  of  freezing  and  thawing  during 
a  portion  of  a  winter  were  very  different  from  those  resulting 
from  Brard's  method,  owing  to  the  action  of  the  salts  being 
chemical  as  well  as  mechanical. 

35.  By  the  absorption  of  water,  stones  become  softer  and 
more  friable.  The  materials  for  road  coverings  should  be 
selected  from  those  stones  which  absorb  least  water,  and  are 
also  hard  and  not  brittle.  Granite,  and  its  varieties,  lime- 
stone, and  common  sand-stone,  do  not  make  good  road  mate- 
rials of  broken  stone.  All  the  hornblende  rocks,  porphyry, 
compact  feldspar,  and  the  quartzose  rock  associated  with 
graywacke,  furnish  good,  durable  road  coverings.  The  fine- 
grained granites  which  contain  but  a  small  proportion  of  mica, 
the  line-grained  silicious  sand-stones  which  are  free  from  clay, 
and  carbonate  of  lime,  form  a  durable  material  when  used  in 
blocks  for  paving.  Mica  slate,  talcose  slate,  hornblende  slate, 
some  varieties  of  gneiss,  some  varieties  of  sand-stone  of  a 
slaty  structure,  and  graywacke  slate,  yield  excellent  materials 
for  flag-stone. 

36.  The  influence  of  locality  on  the  durability  of  stone  is 
very  marked.  Stone  is  observed  to  wear  more  rapidly  in 
cities  than  in  the  country ;  and  the  stone  in  those  parts  of  edi- 
fices exposed  to  the  prevailing  rains  and  winds,  soonest  exhib- 
its signs  of  decay.  The  disintegration  of  the  stratified  stones 
placed  in  a  wall  is  mainly  effected  by  the  position  which  the 
strata  or  quarry  hed  receives,  with  respect  to  the  exposed  sur- 
face ;  proceeding  faster  when  the  faces  of  the  strata  are  ex- 
posed, than  in  the  contrary  position. 

37.  EFFECTS  OF  HEAT,— Stones  which  resist  a  high 
degree  of  heat  without  fusing  are  used  for  lining  furnaces, 


16 


CIVIL  ENGmEEEINO. 


and  are  termed  fire-stones.  A  ffood  fire-stone  should  not  only 
be  infusible,  but  also  not  liable  to  crack  or  exfoliate  from 
heat.  Stones  that  contain  lime,  or  magnesia,  except  in  the 
form  of  silicates,  are  usually  unsuitable  for  fire-stones.  Some 
porous  silicious  limestones,  as  well  as  some  gypsous  silicious 
rocks,  resist  moderate  degrees  of  heat.  Stones  that  contain 
much  potash  are  very  fusible  under  high  temperatures,  run- 
ning into  a  glassy  substance.  Quartz  and  mica,  in  various 
combinations,  furnish  a  good  fire-stone ;  as,  for  example,  finely 
granular  quartz  with  thin  layers  of  mica,  mica  slate  of  the 
same  structure,  and  some  kinds  of  gneiss  which  contain  a 
large  proportion  of  arenaceous  quartz.  Several  varieties  of 
sand-stone  make  a  good  lining  for  furnaces.  They  are  usual- 
ly those  varieties  which  are  free  from  feldspar,  somewhat 
porous,  and  are  uncrystallized  in  the  mass.  Talcose  slate  like- 
wise furnishes  a  good  fire-stone. 

38.  RESISTANCE  TO  ATTRITION.— Hardness  is  an 
essential  quality  in  stone  exposed  to  wear  from  the  attrition 
of  hard  bodies.  Stones  selected  for  paving,  flagging,  and 
steps  for  stairs,  should  be  hard,  and  of  a  grain  sufticiently 
coarse  not  to  admit  of  becoming  very  smooth  under  the  action 
to  which  they  are  submitted.  As  great  hardness  adds  to  the 
difticulty  of  working  stone  with  the  chisel,  and  to  the  coat  of 
the  prepared  material,  builders  prefer  the  softer  ov  free-stones^ 
such  as  the  limestones  and  sand-stones,  for  most  building  pur- 
poses. The  following  are  some  of  the  results,  on  this  point, 
obtained  from  experiment : 

Table  shoiving  the  result  of  experiments  made  under  the  di- 
rection of  Mr,  ^ValJcer,  on  the  wear  of  different  stones  in 
the  tramway  on  the  Commercial  Itoad^  London^  from 
^Ith  March^  1830,  24z^A  August^  1831,  heing  a  period  of 
seventeen  months.    Transactions  of  Civil  Engineers,  vol.  1. 


Name  of  stone. 

Slip,  area 
in  feet. 

Original 

weight. 

Loss  of 
weiglit  by 
woiir. 

Loss  per 
sup.  foot. 

Relative 
losses. 

cwt. 

qrs. 

lbs. 

Guernsey    .  . 

4.734 

7 

1 

12.75 

4.50 

0.951 

1.000 

Herme  .... 

5.250 

7 

3 

24.25 

5.50 

1.048 

1.102 

Budle  .... 

C.3;36 

9 

0 

15.75 

7.75 

1.223 

1.286 

Peterhead  (blue)  . 

3.484 

4 

1 

7.50 

6.25 

1.795 

1.887 

Heytor       .     .  . 

4.313 

6 

0 

15.25 

8.25 

1.915 

2.014 

Aberdeen  (red) 

5.375 

7 

2 

11.50 

11.50 

2.139 

2.249 

Dartmoor  . 

4.500 

6 

2 

25.00 

12.50 

2.778 

2.921 

Aberdeen  (blue)  , 

4.833 

6 

2 

16.00 

14.75 

3.058 

3.216 

BUILDING  MATERIALS. 


17 


The  Commercial  Road  stoneway  consists  of  two  parallel  lines 
of  rectangular  tramstones,  18  inches  wide  by  12  inches  deep, 
and  jointed  to  each  other  endwise,  for  the  wljeels  to  travel  on, 
with  a  common  street  pavement  between  for  the  horses. 

The  following  table  gives  the  results  of  some  experiments 
on  the  wear  or  a  fine-grained  sand-stone  pavement,  by  M. 
Coriolis,  during  eight  years,  upon  the  paved  road  from  Paris 
to  Toulouse,  the  carriage  over  which  is  about  500  tons  daily, 
published  in  the  Annates  des  Ponts  et  Ohausees,  for  March 
and  April,  1834: 


Weight  of  a 
cubic  foot 

Volume  of  water  absorbed  by  the 
dry  stone  after  one  day's  im- 
mersion, compared  with  that  of 
the  stone. 

Mean  annual 
wear. 

1581bs. 

Neglected  as  insensible. 

0.1023  inch. 

154  " 

0.1063  " 

156  " 

u 

0.1299  " 

150  " 

1^8  ill  volume. 

0.2126  " 

148 

0.2677  " 

M.  Coriolis  remarks,  that  the  weight  of  water  absorbed  af- 
fords one  of  the  best  indications  of  the  durability  of  the  fine- 
grained sand-stones  used  in  France  for  pavements.  An 
equally  good  test  of  the  relative  durability  of  stones  of  the 
same  kind,  M.  Coriolis  states,  is  the  more  or  less  clearness  of 
sound  given  out  by  striking  the  stone  with  a  hammer. 

The  following  results  are  taken  fi'om  an  article  by  Mr. 
James  Frost,  Civ.  Engineer,  inserted  in  the  Journal  of  the 
Franhlin  Institute  for  Oct.  1835,  on  the  resistance  of  various 
substances  to  abrasion.  The  substances  were  abraded  against 
a  piece  of  white  statuary  marble,  which  was  taken  as  a  stand- 
ard, represented  by  100,  by  means  of  fine  emery  and  sand. 
The  relative  resistance  was  calculated  from  the  weight  lost  by 
each  substance  during  the  operation. 

Comparative  Resistance  to  Abrasion. 


Aberdeen  granite  ,   ^980 

Hard  Yorkshire  paving  stone   827 

Italian  black  marble   260 

Kilkenny  black  marble   110 

Stntuary  Marble   100 

Old  Portland  stone   79 

Roman  Cement  stone   69 

Fine-grained  Newcastle  grindstone   53 

Stock  brick   34 

Coarse-grained  Newcastle  grindstone   14 

Bath  stone   13 

2 


18 


CIVIL  ENGINEEBING. 


11. 

LIME, 

38.  CLASSIFICATION  OF  LIME.— Considered  as  a 
building  material,  lime  is  now  usually  divided  into  three  prin- 
cipal classes :  Common  or  Air  lime.  Hydraulic  lime,  and  Ily- 
drauliCy  or  Water  cement. 

39.  Common,  or  air  lime,  is  so  called  because  the  paste 
made  from  it  with  water  will  harden  only  in  the  air. 

40.  Hydraulic  lime  and  hydraulic  cement  both  take  their 
name  from  hardening  under  water.  The  former  differs  from 
the  latter  in  two  essential  points.  It  slakes  thoroughly,  like 
common  lime,  when  deprived  of  its  carbonic  acid,  and  it  does 
not  harden  promptly  under  water.  Hydraulic  cement,  on  the 
contrary,  does  not  slake,  and  usually  hardens  very  soon. 

41.  Our  nomenclature,  with  regard  to  these  substances,  is 
still  quite  defective  for  scientific  arrangement.  For  the  lime- 
stones which  yield  hydraulic  lime  when  completely  calcined, 
also  give  an  hydraulic  cement  when  deprived  of  a  portion  only 
of  their  carbonic  acid ;  and  other  limestones  yield,  on  calci- 
nation, a  result  which  can  neither  be  termed  lime  nor  hydraulic 
cement,  owing  to  its  slaking  very  imperfectly,  and  not  retain- 
ing the  hardness  which  it  quickly  takes  w^hen  first  placed  un- 
der water. 

M.  Yicat,  whose  able  researches  into  the  properties  of  lime 
and  mortars  are  so  well  known,  has  proposed  to  apply  the  term 
cement  limestones  {calcaires  d  ciment)  to  those  stones  which, 
when  completel}^  calcined,  yield  hydraulic  cement,  and  which 
under  no  degree  of  calcination  will  give  hydraulic  lime.  For 
the  limestones  which  yield  hydraulic  lime  when  completely 
calcined,  and  which,  when  subjected  to  a  degree  of  heat  insuf- 
ficient to  drive  off  all  their  carbonic  acid,  yield  hydraulic  ce- 
ment, he  proposes  to  retain  the  name  hydraulic  limestones ; 
and  to  call  the  cement  obtained  from  their  incomplete  calci- 
nation under-burnt  hydraulic  cement  {ciments  dHncuits),  to 
distinguish  it  from  that  obtained  from  the  cement  stone.  With 
respect  to  those  limestones  which,  by  calcination,  give  a  result 
that  partakes  partly  of  the  properties  both  of  limes  and  ce- 
ments, he  proposes  for  them  the  name  of  dividing  limes  [chaux 
limites.) 

The  terms and  meager  are  also  applied  to  limes;  owing 
to  the  difference  in  the  quality  of  the  paste  obtained  from 
them  with  the  same  quantity  of  water.    The  fat  limes  give  a 


LDOJS. 


19 


paste  which  is  unctuous  both  to  the  sight  and  touch.  The  meager 
limes  yield  a  thin  paste.  These  names  were  of  some  impor- 
tance when  first  introduced,  as  they  served  to  distinguish  com- 
mon from  hydrauhc  lime,  the  former  being  always  fat,  the 
latter  meager;  but,  later  experience  having  shown  that  all 
meager  limes  are  not  hydraulic,  the  terms  are  no  longer  of 
use,  except  to  designate  qualities  of  the  paste  of  limes. 

42.  Hydraulic  Limes  and  Cements.  The  limestones 
which  yield  these  substances  are  either  argillaceous^  or  mag- 
nesian^  or  argillo-magnesian.  The  products  of  their  calcina- 
tion vary  considerably  in  their  hydraulic  properties.  Some 
of  the  hydraulic  limes  harden,  or  set  very  slowly  under  water, 
while  others  set  rapidly.  The  hydraulic  cements  set  in  a  very 
short  time.  This  diversity  in  the  hydraulic  energy  of  the  ar- 
gillaceous limestones  arises  from  the  variable  proportions  in 
which  the  lime  and  clay  enter  into  their  composition. 

43.  M.  Petot,  a  civil  engineer  in  the  French  service,  in  an 
able  work  entitled  Becherches  sur  la  Chauffournerie,  gives 
the  following  table,  exhibiting  these  combinations,  and  the 
results  obtained  from  their  calcination. 


Lime. 

Clay. 

Eesiilting  products. 

DistinctiTe  characters  of  the  products. 

w 

0 

Very  fat  lime. 

Incapable  of  hardening  in  water. 

90 

10 

Lime  a  little  hydraulic. 

f  Slakes  like  pure  lime,  when 
\     properly  calcined,  and  hard- 

80 

20 

do.   quite  hydraulic. 

70 

30 

do.  do. 

(     ens  under  water. 

60 

40 

Plastic,  or  hydraulic  cement. 

L  Does  not  slake  under  any  cir- 

50 

50 

do. 

•<     cumstances,  and  hardens  un- 

40 

60 

do. 

(     der  water  with  rapidity. 

30 

70 

Calcareous  puzzolano  (brick). 

i  Does  not  slake  nor  harden  un- 

20 

80 

do.  do. 

\     der  water,,  unless  mixed  with 

10 

90 

do.  do. 

(     a  fat  or  an  hydraulic  lime. 

0 

100 

Puzzolano  of  pure  clay  do. 

Same  as  the  preceding. 

44.  The  most  celebrated  European  hydraulic  cements  are 
obtained  from  argillaceous  limestones,  which  vary  but  slightly 
.  in  their  constituent  elements  and  properties.    The  following 
table  gives  the  results  of  analyses  to  determine  the  relative 
proportions  of  lime  and  clay  in  these  cements. 


20  CIVIL  ENGmEEEIKO. 


Table  of  Foreign  Hydraulic  Cements,  showing  the  relative 
jpTojportions  of  Clay  and  Lime  contained  in  them. 


English,  {commonly  known  as  Parker's,  or  Roman  cement) . 
French,  (made  from  Boulogne  pebbles)  

Do.  \PouiUy)  

Do.  do  

Do.      {Baye)   , 

Bussian  , 


The  hydraulic  cements  used  in  England  are  obtained  from 
various  localities,  and  differ  but  little  in  the  relative  propor- 
tions of  lime  and  clay  found  in  them.  Parker's  cement,  so 
called  from  the  name  of  the  person  who  first  introduced  it,  is 
obtained  by  calcining  nodules  of  sejptaria.  The  composition 
of  these  nodules  is  the  same  as  that  of  the  Boulogne  jyebhles 
found  on  the  opposite  coast  of  France.  The  stones  which 
furnish  the  English  and  French  hydraulic  cements  contain 
but  a  very  small  amount  of  magnesia. 

45.  A  hydraulic  cement  known  as  natural  Portland  cement 
is  manufactured  in  France,  at  Boulogne,  where  the  stone, 
which  is  very  soft,  is  found  underlying  the  strata  which  fur- 
nish the  Boulogne  pebbles. 

•  46.  The  best  known  hydraulic  cements  of  the  United  States 
are  manufactured  in  the  State  of  New  York.  The  following 
analyses  of  some  of  the  hydraulic  limestoues,  from  the  most 
noted  localities,  published  in  the  Geological  Report  of  the 
State  of  New  York,  1839,  are  given  by  Dr.  Beck. 

Analysis  of  the  Manlius  Hydraulic  Limestone. 


Carbonic  acid   39.80 

Lime  2G.24 

Magnesia   18.80 

Silica  and  alumina  13.50 

Oxide  of  ii'on   1.25 

Moisture  and  loss   1.41 


100.00 


This  stone  belongs  to  the  same  bed  which  yields  the  hy- 
draulic cement  obtained  near  Kingston,  in  Upper  Canada. 


LIMES. 


21 


Analysis  of  the  Chittenango  Hydraulic  Limestone^  hefore 
and  after  calcination. 


Unburnt. 

39.33 
25.00 
17.83 
11.76 
2.73 
1.50 
1.50 

Carbonic  acid  and  moisture, . 

Alumina  and  oxide  of  iron . . 

100.00 

Carbonic  acid. . . 

Lime  

Magnesia  

Silica  

Alumina  

Peroxide  of  iron 
Moisture  


Burnt. 


10.90 
39.50 
22.27 
16.56 
10.77 

100.00 


Analysis  of  the  HydroAilic  Limestone  from  Ulster  Co.^ 
along  the  line  of  the  Delaware  and  Hudson  Canal,  hefore 
and  after  burning. 


Unburnt. 

Burnt. 

34.20 
25.50 
12.35 
15.37 
9.13 
2.25 
1.20 

5 

37.60 
16.65 
22.75 
13.40 
3.3,0 
1.30 

100.00 

100.00 

The  hydraulic  cement  from  this  last  locality  has  become 
generally  well  known,  having  been  successfully  used  for  most 
of  the  military  and  civil  public  works  on  the  sea-board. 

From  the  results  of  the  analyses  of  all  the  above  lime- 
stones, it  appears  that  the  proportions  of  lime  and  clay 
contained  in  them  place  them  under  the  head  of  hydraulic 
cements,  according  to  the  classification  of  M.  Petot.  They 
do  not  slake,  and  they  all  set  rapidly  under  water. 

47.  The  discovery  of  the  hydraulic  properties  of  certain 
magnesian  limestones  is  of  recent  date,  and  is  due  to  M. 
Yicat,  who  first  drew  attention  to  the  subject.  M.  Yicat 
inclines  to  the  opinion  that  magnesia  alone,  without  the 
presence  of  some  clay,  will  yield  only  a  feeble  hydraulic 
lime.  He  states,  that  he  has  never  been  able  to  obtain  any 
other,  from  proceeding  synthetically  with  common  lime  and 
magnesia ;  and  that  he  knows  of  no  well-authenticated  in- 
stance in  which  any  of  the  dolomites,  either  of  the  primitive 
or  transition  formations,  have  yielded  a  good  hydraulic  lime. 
The  stones  from  these  formations,  he  states,  are  devoid  of 


22 


CIVIL  ENGINEEEINa. 


clay ;  being  very  pure  crystalline  carbonates,  or  else  contain 
silex  only  in  the  state  of  fine  sand.  From  M.  Yicat's  experi- 
ments it  is  rendered  certain  that  carbonate  of  magnesia  in 
combination  with  carbonate  of  lime,  in  proportion  of  40  parts 
of  the  latter  to  from  30  to  40  of  the  former,  will  produce  a 
feebly  hydraulic  lime,  which  does  not  appear  to  increase  in 
hardness  after  it  has  once  set ;  but  that,  with  the  same  -pro- 
portions,  some  hundredths  of  clay  are  requisite  to  give 
hydraulic  energy  to  the  compound.  This  proportion  of  "clay 
M.  Yicat  supposes  may  cause  the  formation  of  triple  hydro- 
silicates  of  lime,  alumina,  and  magnesia,  having  all  the 
characteristic  properties  of  good  hydraulic  lime. 

48.  The  hydraulic  properties  of  the  magnesian  limestones 
of  the  United  States  were  noticed  by  Professor  W.  B.  Rogers, 
who,  in  his  Re])OTt  of  the  Geological  Survey  of  Virgtnia, 
1838,  has  given  the  following  analyses  of  some  of  the  stones 
from  different  localities. 


No.  1. 

No,  2. 

No.  3. 

No.  4 

5o.80 

53.23 

48.20 

55.03 

39.20 

41.00 

35.76 

24.16 

1.50 

0.80 

1.20 

2.60 

2.50 

2.80 

12.10 

15.30 

Water  

0.40 

0.40 

2.73 

1.20 

0.60 

1.77 

0.01 

1.71 

100.00 

100  00 

100.00 

100.00 

The  limestone  No.  1  of  the  above  table  is  from  Sheppards- 
town  on  the  Potomac,  in  Yirginia ;  it  is  extensively  manu- 
factured for  hydraulic  cement.  No.  2  is  from  the  Natural 
Bridge,  and  banks  of  Cedar  Creek,  Yirginia ;  it  makes  a  good 
hydraulic  cement.  No.  3  is  from  New  York,  and  is  extensively 
burnt  for  cement.  No.  4  is  from  Louisville,  Kentucky ;  said 
to  make  a  good  cement. 

49.  M.  Yicat  states,  that  a  magnesian  limestone  of  France, 
containing  the  following  constituents,  lime  40  parts,  magnesia 
21,  and  silicia  21,  yields  a  good  hydraulic  cement ;  and  he 

fives  the  following  analysis  of  a  stone  which  gives  a  good 
ydraulic  lime. 

Carbonate  of  lime   50.60 

Carbonate  of  magnesia   42.00 

Silicia   5.00 

Alumina   2.00 

Oxide  of  iron   0. 40 


100.00 


LIMES, 


23 


By  comparing  the  constituents  of  these  last  two  stones  with 
the  analyses  of  the  cement-stones  of  New  York,  and  the  mag- 
nesian  hydraulic  limestones  of  Prof.  Rogers,  it  will  be  seen, 
that  they  consist,  respectively,  of  nearly  the  same  combina- 
tions of  lime,  magnesia,  and  silica. 

Although  not  brought  out  in  the  analysis  of  the  preceding 
stones,  there  is  probably  none  in  which  the  alkaline  salts  do 
not  occur,  and,  in  some,  of  sufficient  amount  to  injure  mortar 
made  from  them,  by  their  efflorescence. 

50.  PHYSICAL  CHARACTERS  AND  TESTS  OF  HY- 
DRAULIC LIMESTONES.  The  simple  external  characters 
of  a  limestone,  as  color,  texture,  fracture,  and  taste,  are  in- 
sufficient to  enable  a  person  to  decide  whether  it  belongs  to 
the  hydraulic  class ;  although  they  assist  conjecture,  particu- 
larly if  the  rock,  from  which  the  specimen  is  taken,  is  found 
in  connection  with  the  clay  deposits,  or  if  it  belong  to  a 
stratum  whose  general  level  and  characteristics  are  the  same 
as  the  argillo-magnesian  rocks.  These  rocks  are  generally 
some  shade  of  drab,  or  of  gray,  or  of  a  dark  grayish-blue ; 
have  a  compact  texture ;  fracture  even  or  conchoidal ;  with  a 
clayey  or  earthy  smell  and  taste.  Although  the  hydraulic 
limestones  are  usually  colored,  still  it  may  happen  that  the 
stone  may  be  of  a  pure  white,  arising  fi'om  the  combination 
of  lime  with  a  pure  clay.  * 

The  difficulty  of  pronouncing  upon  the  class  to  which  a 
limestone  belongs,  from  its  physical  properties  alone,  renders 
it  necessary  to  resort  to  a  chemical  analysis,  and  even  to  direct 
experiment  to  decide  the  question. 

51.  A  prejudice  exists  among  lime  manufacturers  and 
builders  in  favor  of  the  dark-colored  products  of  calcined 
hydraulic  limestones,  but  without  any  foundation,  so  far  as 
experiment  goes,  as  some  of  the  most  celebrated  cements  are 
light  colored.  As  a  general  rule,  a  dark- colored  material  is 
an  unfavorable  sign,  as  showing  the  presence  of  some  foreign 
ingredient. 

52.  In  making  a  complete  chemical  analysis  of  a  lime- 
stone, more  skill  in  chemical  manipulations  is  requisite  than 
engineers  usually  possess  ;  but  a  person  who  has  the  ordinary 
elementary  knowledge  of  chemistry  can  readily  ascertain  the 
quantity  of  clay  or  of  magnesia  contained  in  a  limestone,  and 
from  these  two  elements  can  pronounce,  with  tolerable  cer- 
tainty, upon  its  hydraulic  properties.  To  arrive  at  this  con- 
clusion, a  small  portion  of  the  stone  to  be  tested — about  five 
drachms — is  taken  and  reduced  to  a  powder ;  this  is  placed 


24 


CIVIL  ETTGnTEEEHTG. 


in  a  capsule,  or  an  ordinary  watch  crystal,  and  slightly  diluted 
muriatic  acid  is  poured  over  it  until  it  ceases  to  effervesce. 
The  capsule  is  then  gently  heated,  and  the  liquor  evaporated, 
until  the  residue  in  the  capsule  has  acquired  the  consistence 
of  thin  paste.  This  paste  is  thrown  into  a  pint  of  pure  water 
and  well  shaken  up,  and  the  mixture  is  then  filtered.  The 
residue  left  on  the  filtering  paper  is  thoroughly  dried,  by 
bringing  it  to  a  red  heat ;  this  bein^  weighed  will  give  the 
clay^  or  insoluble  matter,  contained  in  the  stone.  It  is  import- 
ant to  ascertain  the  state  of  mechanical  division  of  the  in- 
soluble matter  thus  obtained  ;  for  if  it  be  wholly  granular,  the 
stone  will  not  yield  hydraulic  lime.  The  granular  portion 
must  therefore  be  carefully  separated  from  the  other  before 
the  latter  is  dried  and  weighed. 

53.  If  the  sample  tested  contains  magnesia,  an  indication 
of  this  will  be  given  by  the  slowness  with  which  the  acid  acts; 
if  the  quantity  of  magnesia  be  but  little,  the  solution  will  at 
first  proceed  rapidly  and  then  become  more  sluggish.  To 
ascertain  the  quantity  of  magnesia,  clear  lime-water  must  be 
added  to  the  filtered  solution  as  long  as  any  preci^^itate  is 
formed,  and  this  precipitate  must  be  quickly  gathered  on  fil- 
tering paper,  and  then  be  washed  with  pure  water.  The  resi- 
due from  this  washing  is  the  magnesia.  It  must  be  thoroughly 
dried  before  being  weighed,  to  ascertain  its  proportion  to  the 
clay. 

54.  Having  ascertained,  by  the  preceding  analysis,  the 
probable  hydraulic  energy  of  the  stone,  a  sample  of  it  should 
also  be  submitted  to  direct  experiment.  This  may  be  likewise 
done  on  a  small  scale.  A  sample  of  the  stone  must  be  re- 
duced to  fragments  about  the  size  of  a  walnut.  A  crucible, 
perforated  with  holes  for  the  free  admission  of  air,  is  filled  with 
these  fragments,  and  placed  over  a  fire  sufliciently  powerful 
to  drive  olf  the  carbonic  acid  of  the  stone.  The  time  for 
effecting  this  will  depend  on  the  intensity  of  the  heat.  "Wlien 
the  heat  has  been  applied  for  three  or  four  hours,  a  small  por- 
tion of  the  calcined  stone  may  be  tried  with  an  acid,  and  the 
degree  of  the  calcination  may  be  judged  of  by  the  more  or 
less  copiousness  of  the  effervescence  that  ensues.  If  no 
effervescence  takes  place,  the  operation  may  be  considered 
completed.  The  calcined  stone  should  be  tried  soon  after  it 
has  become  cold ;  otherwise,  it  should  be  kept  in  a  glass  jar 
made  as  air  tight  as  practicable  until  used. 

55.  When  the  calcined  stone  is  to  be  tried,  it  is  first  slaked 
by  placing  it  in  a  small  basket,  which  is  immersed  for  five  or 
six  seconds  in  pure  water.     The  stone  is  emptied  fi-om  the 


LIMES. 


25 


basket  so  soon  as  the  water  has  drained  off,  and  is  allowed  to 
stand  until  the  slaking  is  terminated.  This  process  will  pro- 
ceed more  or  less  rapidly,  according  to  the  quality  of  the  stone, 
and  the  degree  of  its  calcination.  In  some  cases,  it  will  be 
completed  in  a  few  minutes ;  in  others,  portions  only  of  the 
stone  will  fall  to  powder,  the  rest  crumbling  into  lumps  which 
slake  very  sluggishly ;  while  other  varieties,  as  the  true  cement 
stones,  give  no  evidence  of  slaking.  If  the  stone  slakes  either 
completely  or  partially,  it  must  be  converted  into  a  paste  of 
the  consistence  of  soft  putty,  being  ground  up  thoroughly,  if 
necessary,  in  an  iron  mortar.  The  paste  is  made  into  a  cake, 
and  placed  on  the  bottom  of  an  ordinary  tumbler,  care  being 
taken  to  make  the  diameter  of  the  cake  the  same  as  that  of 
the  tumbler.  The  vessel  is  filled  with  water,  and  the  time  of 
immersion  noted.  If  the  lime  is  only  moderately  hydraulic, 
it  will  have  become  hard  enough  at  the  end  of  fifteen  or  twen- 
ty days,  to  resist  the  pressure  of  the  finger,  and  will  continue 
to  harden  slowly,  more  particularly  from  the  sixth  or  eighth 
month  after  immersion  ;  and  at  the  end  of  a  year  it  will  have 
acquired  the  consistency  of  hard  soap,  and  will  dissolve  slowly 
in  pure  water.  A  fair  hydraulic  lime  will  have  hardened  so 
as  to  resist  the  pressure  of  the  finger,  in  about  six  or  eight  days 
after  immersion,  and  will  continue  to  grow  harder  until  from 
six  to  twelve  months  after  immersion ;  it  will  then  have  ac- 
quired the  hardness  of  the  softest  calcareous  stones,  and  will 
be  no  longer  soluble  in  pure  water.  AVhen  the  stone  is  emi- 
nently hydraulic,  it  will  have  become  hard  in  from  two  to  four 
days  after  immersion,  and  in  one  month  it  will  be  quite  hard 
and  insoluble  in  pure  water;  after  six  months,  its  hardness 
will  be  about  equal  to  the  more  absorbent  calcareous  stones ; 
and  it  will  sjDlinter  from  a  blow,  presenting  a  slaty  fracture. 

As  the  hydraulic  cements  do  not  slake  perceptibly,  the  burnt 
stone  must  first  be  reduced  to  a  fine  powder  before  it  is  made 
into  a  paste.  The  paste,  when  kneaded  between  the  fingers, 
becomes  warm,  and  will  generally  set  in  a  few  minutes,  either 
in  the  open  air  or  in  water.  Hydraulic  cements  are  far  more 
sparingly  soluble  in  pure  water  than  the  hydraulic  lime ;  and 
the  action  of  pure  water  upon  them  ceases,  apparently,  after 
a  few  weeks'  immersion  in  it. 

56.  Caleination  of  Limestone.  The  effect  of  heat  on 
lime-stones  varies  with  the  constituent  elements  of  the  stone. 
The  pure  limestones  will  stand  a  high  degree  of  temperature 
without  fusing,  losing  only  their  carbonic  acid  and  water. 
The  impure  stones  containing  silica  fuse  completely  under  a 
great  heat,  and  become  more  or  less  vitrified  when  the  tem- 


26 


CrVTL  ENGINEERmG. 


perature  much  exceeds  a  red  heat.  The  action  of  heat  on  the 
impure  limestones,  besides  driving  off  their  carbonic  acid  and 
water,  modilies  the  relations  of  their  other  chemical  constitu- 
ents. The  argillaceous  stones,  for  example,  yield  an  insoluble 
precipitate  when  acted  on  by  an  acid  before  calcination,  but 
are  perfectly  soluble  afterwards,  unless  the  silex  they  contain 
happens  to  be  in  the  form  of  grains. 

57.  The  calcination  of  the  hydraulic  limestones,  from  their 
fusible  nature,  requires  to  be  conducted  with  great  care  ;  for, 
if  not  pushed  far  enough,  the  under-burnt  portions  will  not 
slake ;  and,  if  carried  too  far,  the  stone  becomes  dead  or 
sluggish  ;  slakes  very  slowly  and  imperfectly  at  first ;  and,  if 
used  in  this  state  for  masonry,  may  do  injury  by  the  swelling 
which  accompanies  the  after-slaking. 

58.  The  more  or  less  facility  with  which  the  impure  lime- 
stones can  be  burned  depends  upon  several  causes;  as  the 
compactness  of  the  stone,  the  size  of  the  fragments  submit- 
ted to  heat,  and  the  presence  of  a  current  of  air,  or  of  aque- 
ous vapor.  The  more  compact  stones  yield  their  carbonic 
acid  less  readily  than  those  of  an  opposite  texture.  Stones 
w^hich,  when  broken  into  very  small  lumps,  can  be  calcined 
under  the  red  heat  of  an  ordinary  fire  in  a  few  hours,  will  re- 
quire a  far  greater  degree  of  temperature,  and  for  a  much 
longer  period,  when  broken  into  fragments  of  six  or  eight 
inches  in  diameter.  This  is  particularly  the  case  with  the  im- 
pure limestones,  which,  when  in  large  lumps,  vitrify  at  the 
surface  before  the  interior  is  thoroughly  burnt. 

59.  If  a  current  of  vapor  is  passed  over  the  stone  after  it  has 
commenced  to  give  off  its  carbonic  acid,  the  remaining  por- 
tion of  the  gas  which,  under  ordinary  circumstances,  is  expelled 
with  great  difficulty,  particularly  near  the  end  of  the  process 
of  calcination,  will  be  carried  off  much  sooner.  The  infiuence 
of  an  aqueous  current  is  attributed,  by  M.  Gay-Lussac,  purely 
to  a  mechanical  action,  by  removing  the  gas  as  it  is  evolved, 
and  his  experiments  go  to  show  that  a  like  effect  is  produced 
by  an  atmospheric  current.  In  burning  the  impure  lime- 
stones, however,  an  aqueous  current  produces  the  farther 
beneficial  effect  of  preventing  the  vitrification  of  the  stone 
when  the  tenq^erature  has  become  too  elevated ;  but  as  the 
vapor,  on  coming  in  contact  with  the  heated  stone,  carries  off 
a  large  portion  of  the  heat,  this,  together  with  the  latent  heat 
contained  in  it,  may  render  its  use  in  some  cases  far  from 
economical. 

60.  Wood,  charcoal,  peat,  the  bituminous  and  the  anthra- 
cite coals  are  used  for  fuel  in  lime-burning.    M.  Yicat  states, 


LIME-KILNS. 


27 


that  wood  is  the  best  fuel  for  burning  hydraulic  limestones  ; 
that  charcoal  is  inferior  to  bituminous  coal ;  and  that  the  re- 
sults from  this  last  are  very  uncertain.  When  wood  is  used, 
it  should  be  dry  and  split  up,  to  burn  quickly  and  give  a  clear 
blaze.  The  common  opinion  among  lime-burners,  that  the 
greener  the  fuel  the  better,  and  that  the  limestone  should  be 
watered  before  it  is  placed  in  the  kiln,  is  wrong  ;  as  a  large 
portion  of  the  heat  is  consumed  in  converting  the  water  in  both 
cases  into  vapor.  Coal  is  a  more  economical  fuel  than  wood, 
and  is  therefore  generally  preferred  to  it;  but  it  requires 
particular  care  in  ascertaining  the  proper  quantity  for  use. 


III. 

LIME  KILNS. 

LIME  KILN'S.  Great  diversity  is  met  with  in  the  forms  and 
proportions  of  lime-kilns.  Wherever  attention  has  been  paid 
to  economy  in  fuel,  the  cylindrical,  ovoidal,  or  the  inverted 
conical  form  has  been  adopted.  The  two  first  being  preferred 
for  wood  and  the  last  for  coal. 

^  61.  The  whole  of  the  burnt  lime  is  either  drawn  from  the 
kiln  at  once,  or  else  the  burning  is  so  regulated,  that  fresh 
stone  and  fuel  are  added  as  the  calcined  portions  are  with- 
drawn. The  latter  method  is  usually  followed  when  the  fuel 
used  is  coal.  The  stone  and  coal,  iDroken  into  proper  sizes 
(Fig.  1),  and  in  proportions  determined  by  experiment,  are 


Fig.  1  represents  a  vertical  section  through  the  axis  and  centre  lines  of 
the  entrances  communicating  with  the  interior  of  a  kiln  for  burning 
lime  with  coal. 

A,  solid  masonry  of  the  kiln,  which  is  built  up  on  the  exterior  like  a 
square  tower,  with  two  arched  entrances  at  B,  B  on  opposite  sides. 

C,  interior  of  the  kiln,  lined  with  fire-brick  or  stone. 

D,  ash-pit. 

c,  c,  openings  between  B,  B  and  the  interior  through  which  the  burnt 
lime  is  drawn. 


placed  in  the  kiln  in  alternate  layers ;  the  coal  is  ignited  at 
the  bottom  of  the  kiln,  and  fresh  strata  are  added  at  the  top, 
as  the  burnt  mass  settles  down  and  is  partially  withdrawn  at  the 


28 


CIVIL  ENGINEERING. 


bottom.  Kilns  used  in  this  ^nj  are  csiiled  perpetual  Him  / 
they  are  more  economical  in  the  consumption  of  fuel  than 
those  in  which  the  burning  is  intermitted,  and  which  are,  on 
this  account,  termed  intermittent  kilns.  Wood  may  also  be 
used  as  fuel  in  perpetual  kilns ;  but  not  with  such  economy 
as  coal ;  it  moreover  presents  many  inconveniences,  in  sup- 
plying the  kiln  with  fresh  stone,  and  in  regulating  its  dis- 
charge. The  inverted  conical-shaped  kiln  is  generally  adopted 
for  coal,  and  the  ovoid al-shaped  for  wood. 

62.  Some  care  is  requisite  in  filling  the  the  kiln  with  stone 
when  a  wood  fire  is  used.    A  dome  (Fig.  2)  is  formed  of  the 


Fig.  2  represents  a  vertical  section  through  the  axis  and 
centre  line  of  the  entrance  of  a  lime-kiLn  for  wood. 

A,  solid  masonry  of  the  kiln. 

B,  arched  entrance. 

C,  doorway  for  drawing  kiln  and  supplying  fuel. 

D,  interior  of  kiln. 

E,  dome  of  broken  stone,  shown  by  the  dotted  line. 


largest  blocks  of  the  broken  stone,  which  either  rests  on  the 
bottom  of  the  kiln  or  on  the  ash-grate.  The  lower  diameter 
of  the  dome  is  a  few  feet  less  than  that  of  the  kiln ;  and  its 
interior  is  made  sufficiently  capacious  to  receive  the  fuel  which, 
cut  into  short  lengths,  is  placed  up  endwise  around  the  dome. 
The  stone  is  placed  over  and  around  the  courses  which  form 
the  dome,  the  largest  blocks  in  the  centre  of  the  kiln.  The 
management  of  the  fire  is  a  matter  of  experiment.  For  the 
first  eight  or  ten  hours  it  should  be  carefully  regulated,  in  or- 
der to  bring  the  stone  gradually  to  a  red  heat.  By  apj^lying 
a  higli  heat  at  first,  or  by  any  sudden  increase  of  it  until  the 
mass  has  reached  a  nearly  uniform  temperature,  the  stone  is 
apt  to  shiver,  and  choke  the  kiln,  by  stopping  the  voids  be- 
tween the  courses  of  stone  which  form  the  dome.  After  the 
stone  is  brought  to  a  red  heat,  the  supply  of  fuel  should  be 
uniform  until  the  end  of  the  calcination.  The  practice  some- 
times adopted,  of  abating  the  fire  towards  the  end,  is  bad,  as 
the  last  portions  of  carbonic  acid  retained  by  the  stone,  require 
a  high  degree  of  heat  for  their  expulsion.  The  indications  of 
complete  calcination  are  generally  manifested  by  the  diminu- 
tion which  gradually  takes  place  in  the  mass,  and  which,  at 
this  stage,  is  about  one  sixth  of  the  primitive  volume ;  by  the 
broken  appearance  of  the  stone  w^hich  forms  the  dome,  the 


29 


interstices  between  which  being  also  choked  up  by  fragments 
of  the  bnrnt  stone ;  and  by  the  ease  with  which  an  iron  bar 
may  be  forced  down  through  the  burn  stone  in  the  kiln.  When 
these  indications  of  complete  calcination  are  observed,  the 
kiln  should  be  closed  for  ten  or  twelve  hours,  to  confine  the 
heat  and  finish  the  burning  of  the  upper  strata. 

63.  The  form  and  relative  dimensions  of  a  kiln  for  wood  can 
be  determined  only  by  careful  experiment.  If  too  great 
height  be  given  to  the  mass,  the  lower  portions  may  be  over- 
burned  before  the  upper  are  burned  enough.  The  propor- 
tions between  the  height  and  mean  horizontal  section,  will 
depend  upon  the  texture  of  the  stone ;  the  size  of  the  frag- 
ments into  which  it  is  broken  for  burning ;  and  the  more  or 
less  facility  with  which  it  vitrifies.  In  the  memoir  of  M. 
Petot,  already  cited,  it  is  stated  as  the  result  of  experiments 
made  at  Brest,  that  large-sized  kilns  are  more  economical,  both 
in  the  consumption  of  fuel  and  in  the  cost  of  attendance,  than 
small  ones ;  but  that  there  is  no  notable  economy  in  fuel  wdien 
the  mean  horizontal  section  of  the  kiln  exceeds  sixty  square 
feet. 

64.  The  circular  seems  the  most  suitable  form  for  the  hori- 
zontal sections  of  a  kiln,  both  for  strength  and  economizing 
the  heat.  Were  the  section  the  same  throughout,  or  the  form 
of  the  interior  of  the  kiln  cylindrical,  the  strata  of  stone, 
above  a  certain  point,  Avould  be  very  imperfectly  burned  when 
the  lower  were  enough  so,  owing  to  the  rapidity  with  wdiich 
the  inflamed  gases,  arising  from  the  combustion,  are  cooled  by 
coming  into  contact  with  the  stone.  To  procure,  therefore,  a 
temperature  throughout  the  heated  mass  which  shall  be  nearly 
uniform,  the  horizontal  sections  of  the  kiln  should  gradually 
decrease  from  the  point  where  the  flame  rises,  which  is  near 
the  top  of  the  dome  of  broken  stone,  to  the  to-p  of  the  kiln. 
This  contraction  of  the  horizontal  section,  from  the  bottom 
upward,  should  not  be  made  too  rapidly,  as  the  draft  w^ould 
be  injured,  and  the  capacity  of  the  kiln  too  much  diminished; 
and  in  no  case  should  the  area  of  the  top  openmg  be  less  than 
about  one  fourth  the  area  of  the  section  taken  near  the  top  of 
the  dome.  Tlie  best  manner  of  arranging  the  sides  of  the  kiln, 
in  the  plane  of  the  longitudinal  section,  is  to  connect  the  top 
opening  with  the  horizontal  section  through  the  top  of  the 
dome,  by  an  arc  of  a  circle  whose  tangent  at  the  lower  point 
shall  be  vertical. 

65.  Lime-kilns  are  constructed  either  of  brick  or  of  some 
of  the  more  refractory  stones.  The  walls  of  the  kiln  should 
be  sufficiently  thick  to  confine  the  heat,  and,  when  the  locality 


30 


CmL  ENGINEEEING. 


admits  of  it  they  are  built  into  a  side  hill ;  otherwise,  it  may 
be  necessary  to  use  iron  hoops,  and  vertical  bars  of  iron,  to 
strengthen  the  brick-work.  The  interior  of  the  kiln  should 
be  faced  either  with  good  fire-brick  or  with  fire-stone. 

66.  M.  Petot  prefers  kilns  arranged  with  fire-grates,  and  an 
ash-pit  under  the  dome  of  broken  stone,  for  the  reason  that 
they  give  the  means  of  better  regulating  the  heat,  and  of 
throwing  the  flame  more  in  the  axis  of  the  kiln  than  can  be 
done  in  kilns  without  them.  The  action  of  the  flame  is  thus 
more  uniformly  felt  through  the  mass  of  stone  above  the  top 
of  the  dome,  while  that  of  the  radiated  heat  upon  the  stone 
around  the  dome  is  also  more  uniform. 

67.  M.  Petot  states,  that  the  height  of  the  mass  of  stone 
above  the  top  of  the  dome  should  not  be  greater  than  from 
ten  to  thirteen  feet,  depending  on  the  more  or  less  compact 
texture  of  the  stone,  and  the  more  or  less  ease  with  which  it 
vitrifies.    He  proposes  to  use  kilns  with  two  stories  (Fig.  3), 


Fig.  3  represents  a  vertical  section 
through  the  axis  and  centre  line  of 
the  entrance  of  a  lime-kiln  with  two 
stories  for  wood. 

A,  solid  masonry  of  the  kiln. 

B,  dome  shown  by  the  dotted  line. 

C,  interior  of  lower  story. 

D,  dome  of  upper  story. 

E,  interior  of  upper  story, 
a,  arched  entrance  to  kiln. 

&,  receptacle  for  water  to  furnish  a  cur- 
rent of  aqueous  vapor. 

c,  doorway  for  drawing  kiln,  etc.,  closed 
by  a  firc-proof  door. 

d,  ash-pit  under  fire-grate. 

e,  upper  doorway  for  drawing  kiln,  etc. 


for  the  purpose  of  economizing  the  fuel,  by  using  the  heat 
which  passes  off  from  the  top  of  the  lower  story,  and  would 
otherwise  be  lost,  to  heat  the  stone  in  the  upper  story ;  this 
story  being  arranged  with  a  side-door,  to  introduce  fuel  under 
its  dome  of  broken  stone,  and  complete  the  calcination  when 
that  of  the  stone  in  the  lower  story  is  finished. 

M.  Petot  gives  the  following  general  directions  for  regulat- 
ing the  relative  dimensions  of  the  parts  of  the  kiln.  The 
greatest  horizontal  section  of  the  kiln  is  placed  rather  below 
the  top  of  broken  stone ;  the  diameter  of  this  section  being 
1.82,  the  diameter  of  the  grate.    The  height  of  the  dome 


LIME-KILNS. 


81 


above  the  grate  is  from  3  to  6  feet,  according  to  the  quantity 
of  fuel  to  be  consumed  hourly.  The  bottom  of  the  kiln,  on 
which  the  piers  of  the  dome  rest,  is  from  4  to  6  inches  above 
the  top  of  the  grate ;  the  diameter  of  the  kiln  at  this  point 
being  about  2  feet  9  inches  greater  than  that  of  the  grate. 
The  diameter  of  the  horizontal  section  at  top  is  0.63  the  di- 
ameter of  the  greatest  horizontal  section.  The  horizontal  sec- 
tions of  the  kiln  diminish  from  the  section  near  the  top  of  the 
dome  to  the  top  and  bottom  of  the  kiln ;  the  sides  of  the  kiln 
receiving  the  form  shown  in  Fig.  3:  the  object  of  contracting 
the  kiln  towards  the  bottom  being  to  allow  the  stone  near  the 
bottom  to  be  thoroughly  burned  by  the  radiated  heat.  The  grate 
is  formed  of  cast-iron  bars  of  the  usual  form,  the  area  of  the 
spaces  betwen  the  bars  being  one  fourth  the  total  area  of  the 
grate.  The  bottom  of  the  ash-pit,  w^hich  may  be  on  the  same 
level  as  the  exterior  ground,  is  placed  18  inches  below  the 
grate ;  and  at  the  entrance  of  the  ash-pit  is  placed  a  reservoir 
for  water,  about  18  inches  in  depth,  to  furnish  an  aqueous 
current.  The  draft  through  the  grate  is  regulated  by  a  lateral 
air  channel  to  the  ash-pit,  w^hich  can  be  totally  or  partially 
shut  by  a  valve ;  the  area  of  the  cross  section  of  this  channel 
is  one  tenth  the  total  area  of  the  grate.  A  square  opening, 
16  inches  wide,  the  bottom  of  which  is  on  a  level  with 
the  bottom  of  kiln,  leads  to  the  dome  for  the  supply  of  the 
fuel.  This  opening  is  closed  with  a  fire-proof  and  air-tight 
door. 

In  arranging  a  kiln  with  two  stories,  M.  Petot  states,  that 
the  grates  of  the  upper  story  are  so  soon  destroyed  by  the 
heat,  that  it  is  better  to  suppress  them,  and  to  place  the  fuel 
for  completing  the  calcination  of  the  stone  of  this  story  on 
the  top  of  the  burnt  stone  of  the  lower  story. 

68.  Lime  burning  has  become  a  special  branch  of  industry 
in  the  United  States,  in  which  a  large  amount  of  capital  is 
embarked,  so  that  the  engineer  has  now  no  other  concern  in 
the  manufacture  of  this  material  than  to  be  able  to  test  and 
select  from  the  samples  offered  him  to  suit  the  application  he 
intends  making  of  his  material. 

69.  There  are  tw^o  principal  classes  of  lime-kilns  employed 
by  the  manufacturers  of  lime  in  the  United  States.  These 
vary  but  little  from  each  other  in  form  and  dimensions  in  the 
localities  in  which  they  are  used  throughout  the  country. 

70.  The  first  class  belongs  to  the  jperpetual  kilns,  the 
stone  and  fuel,  which  is  usually  bituminous  or  anthracite  coal, 
being  placed  in  the  kiln  in  alternate  layers,  in  proportions 
pointed  out  by  experience,  which  is  fed  in  like  manner  at  the 


32 


CIVIL  ENGINEEEING. 


top  as  the  calcined  stone  is  gradually  dra^vn  out  at  the  bottom. 
In  some  cases  the  chamber  of  these  kilns  is  simply  an  invert- 
ed frustum  of  a  cone  in  form. 


h  d 


Fig.  4  represents  a  section  through  the  axis  of  the 
perpetual  lime-kilns  in  ordinary  use  in  the  United 
States  for  coal  as  the  fuel. 

A,  body  of  the  kiln. 

B,  thimble,  or  lower  frustum. 

C,  D,  draw  pit. 

F,  body  of  the  masonry. 

06,  erf,  sides  of  conical- shaped  kiln. 


71.  In  others  (Tig.  4)  the  body  or  upper  portion  of  the  chamber 
is  cylindrical,  whilst  the  lower  portion  is  an  inverted  conical 
frustum,  the  two  surfaces  being  united  by  an  annular  one 
tangent  to  each. 

72.  The  second  class  is  the  flume  or  furnace  kiln.  In  this 
the  stone  placed  in  the  chamber  of  the  kiln  is  calcined  by  the 
combustion  of  the  fuel,  either  wood  or  coal,  placed  in  furnaces 
near  the  bottom  of  the  chamber.  This  class  may  be  used 
either  as  intermittent  or  perpetual  kilns. 

73.  In  both  classes  the  stone  for  burning  is  broken  into 
lumps,  none  of  which  should  be  over  eight  inches  in  size  in 
any  direction.  In  the  selection  of  the  lumps  great  care  and 
experience  are  required  on  the  part  of  the  kiln  attendants,  in 
order  to  obtain  a  product  of  uniform  quality,  as  admixtures 
of  stones  varying  in  any  important  degree  in  their  constituent 
elements,  particularly  in  those  of  hydraulic  limestones,  may 
so  vitiate  the  results  as  to  render  them  useless  for  hydraulic 
structures. 

74.  In  others  they  are  formed  of  the  frusta  of  two  conical 
surfaces,  as  shown  by  the  dotted  lines  a  h,  c  d,  united  at 
their  larger  bases  (Fig.  4). 

The  diameter  a  c  oi  the  thimble  varies  from  eight  to  ten  feet ; 
the  diameter  at  the  bottom  from  eighteen  inches  to  three 
feet ;  the  height  of  the  thimble  from  seven  to  ten  feet.  The 
upper  diameter  of  the  body  of  the  kiln,  if  conical,  is  about  a 


LIME-KILNS. 


33 


Fig.  7, 


Fig.  5  is  a  horizontal  section  taken  at  CC,  Figs.  6,  7 ;  Fig.  6  is  a  vertical  section  taken  at 
KK,  Fig.  5,  through  the  main  furnace ;  and  Fig.  7  a  vertical  section  through 
A  A  of  the  water  flame  kiln  for  coal.  G-,  body  of  the  masonry. 

H,  H  is  the  cupola  or  body  of  the  kiln. 

I,  wall  dividing  the  cupola,  and  rising  from  bottom  of  kiln  to  a  level  with  the  side-flues, 
J,  wooden  crib  on  top.  K,  furnace  arches.  L,  ash  pit. 

M,  water-pipes  for  supplying  water-pans  c  and  ash  pans/. 
N,  curved  iron  lining  at  bottom  serving  as  a  slide. 

a,  a,  concaves  in  interior  of  cupola. 

b,  b,  gi-ates.  c,  c,  hot- water  coal-pans. 

i,  i,  sight-holes  for  examining  burning  of  body  of  Ume  and  punching  it  dovmwards. 
g,  g,  side  flues. 

2* 


84 


CIVIL  ENGINEERING. 


footless  than  the  lower  ;  if  cylindrical,  the  same  as  the  lower. 
The  height  of  the  body  from  twelve  to  twenty  feet.  The 
draw  door  from  eighteen  inches  to  three  feet.  The  height  of 
the  draw  pit  nine  feet. 

The  body  A  of  the  masonry  is  sometimes  rectangular  and 
sometimes  circular  in  plan,  and  about  six  feet  in  thickness. 
It  is  secured  on  the  outside  either  by  strips  of  wood  let  into 
the  masonry,  or  by  iron  curbs.  The  lining  of  the  kiln  is  of  the 
best  fire-brick. 

The  kiln,  for  burning,  is  filled  with  alternate  layers  of  coal 
and  stone,  those  of  the  latter  not  exceeding  six  inches  in  thick- 
ness. The  fire  is  started  from  beneath,  with  dry  wood.  The 
drawing  of  the  kiln  is  done  two  or  three  times  every  twenty- 
four  hours. 

75.  The  perpetual  draw  water-flame  kilns,  for  both  coal  and 
wood,  patented  by  Mr.  C.  D.  Page,  of  Rochester,  ISTew  York, 
have  met  with  very  general  favor  in  our  large  lime  burning 
localities. 

The  cupola  which  contains  the  burning  lime,  it  will  be  seen, 
is  chiefly  cylindrical,  being  terminated  at  top  and  bottom  by 
conical  frusta. 

•  The  cupola  space  is  six  by  eight  feet  between  the  main  walls 
AA.  The  main  walls  from  out  to  out  are  eighteen  by  twenty 
feet  at  the  base  of  the  kiln  ;  fifteen  by  sixteen  feet  at  the  top ; 
and  forty  feet  high.  The  main  walls  are  strengthened  as  usual 
with  timber  curbs.  The  wooden  crib  at  top,  which  is  strong- 
ly boarded  to  the  height  of  four  feet,  serves  as  a  reservoir  for 
the  raw  stone. 

This  kiln  receives  its  name  from  the  coal  being  first  placed 
in  pans  of  hot  water,  the  steam  from  which  being  decomposed 
facilitates  the  process  of  burning  by  the  decomposition  of  the 
steam. 

76.  HofFman  Kiln.  General  Q.  A.  Gillmore,  of  the  Uni- 
ted States  Corps  of  Engineers,  to  whom  the  profession  is 
already  so  much  indebted  for  his  researches  on  the  limes  and 
cements  in  the  United  States,  has  given  in  his  recent  pam- 
phlet, I^'o.  19,  Professional  Papers^  Corjps  of  Engineers^  U.S. 
Army,  an  account  of  what  is  known  as  the  Hoffman  Kiln^ 
of  which  the  following  is  a  brief  description : — 

This  kiln  (Figs.  8,  9,  10,  11)  consists  of  an  annular  arch,  A, 
A',  the  plan  of  which  may  be  a  circle,  an  oval,  or  as  in  Fig. 
8.  The  height  of  the  arch  being  from  eight  to  nine  feet,  and 
span  from  ten  to  twelve  feet ;  the  middle  line  of  the  chamber 
A  measuring  one  hundred  and  fifty  feet.  This  void  space  is 
termed  the  ourning  chamber. 


LIME-KILNS. 


35 


The  chimney  C,  C  (Figs.  10,  11)  may  stand  in  the  central 
space  B,  B',  or  exterior  to  the  kihi.  In  the  latter  case  a  smoke 
flue  leads  to  it  nnder  the  burning  chamber.  Fourteen  radial 
flues  lead  from  the  burning  chambers  to  the  smoke  chamber, 


Fig.  8. 


Fig.  8.  Horizontal  section  of  kiln  on  A  B,  Fig.  9. 

Fig.  9.  Vertical  section  on  C  D,  Fig.  8. 

Fig.  10.  Elevation  of  chimney. 

Fig.  11.  Section  of  chimney  at  A  B,  Fig.  10. 

A,  A',  Burning  chamber. 

B,  B',  Smoke  chamber. 

C,  C,  Chimney. 

D,  Doorways. 

a,  h.  Lime-stone  in  process  of  bTiming. 
6,  c,        do.  do.       of  cooling. 

c,  d,        do.  do.      of  drawing. 

d,  e,        do.  do.      of  setting  up. 
«,/,        do.  do.      of  drying. 

f.  a,        do.  do.      of  taking  up  waste  heat. 

each  having  a  bell-shaped  damper,  which  can  be  opened  or 
closed  at  pleasure.  There  are  fourteen  arched  doors,  D,  D, 
through  the  outer  wall,  each  five  feet  high,  and  four  feet  wide. 

The  arched  top  of  the  burning  chamber  is  pierced,  at  inter- 
vals of  three  or  four  feet,  with  holes,  five  inches  in  diameter, 
termed  feed-holes,  through  which  fuel  is  supplied  to  the  fires. 


36 


CTVTL  ENGINEEEING. 


They  are  in  number  about  three  hundred,  each  closed  with 
a  bell-shaped  cover  fitting  over  a  rim  or  curb,  and  dipping 
into  sand. 

The  entire  structure  is  of  solid  stone  or  brick  masonry,  and 
covered  with  a  roof. 

The  burning  chamber  is  lined  with  fire-brick  for  burning 
hydraulic  cement. 

77.  Calcination  of  the  stone. — ^When  the  kiln  is  in  opera- 
tion all  the  doorways  (Fig.  8)  numbered  from  1  to  14,  from 
left  to  right  are  kept  closed  with  temjyorary  hrickworJc,  ex- 
cept two  or  three.  Let  the  open  ones  be  1  and  2.  The 
burnt  lime  is  drawn  fi'om  No.  2,  and  raw  stone  taken  in 
at  No.  1  and  piled  up  in  the  burning  chamber,  leaving 
vertical  openings  under  the  feed  holes,  and  horizontal  ones 
under  the  mass  for  the  circulation  of  air  around  the  periphery 
of  the  burning  chamber. 

When  the  kiln  is  going,  all  the  compartments  but  two, 
between  each  two  consecutive  doorways,  are  filled  with  stone, 
in  all  stages,  from  the  raw  to  thoroughly  calcined. 

"  Suppose  compartments  1  and  2  empty,  and  all  the  others 
filled.  No.  3  contains  cement  from  stone  put  in  12  days  ago ; 
No.  4  that  from  stone  put  in  11  days  ago ;  and  so  on  around 
to  compartment  14,  which  was  filled  yesterday.  Separating 
No.  14  from  No.  1  is  a  sheet  iron  partition,  as  nearly  as  pos- 
sible air-tight.  This  partition,  called  the  mct-off,  is  movable. 
Yesterday  it  was  between  13  and  14 ;  to-morrow  it  will  be 
between  1  and  2,  and  so  on,  being  moved  on  one  compart- 
ment each  day.  All  the  dampers  are  closed  to-day  except 
No.  14 ;  yesterday  all  were  closed  except  No.  13  ;  to-morrow 
only  No.  1  will  be  open.  To-day  men  are  removing  burnt 
cement  from  compartment  No.  2,  and  others  are  setting  raw 
stone  in  compartment  No.  1.  Yesterday  they  were  setting 
stone  in  No.  14,  and  removing  cement  from  No.  1.  To- 
morrow they  will  be  removing  cement  from  No.  3,  and  filling 
No.  2  with  raw  stone ;  so  that  every  day  the  setting,  drawing, 
cut-olf,  and  open  damper  advance  one  compartment.  The 
fires  are  in  the  centre  oi  the  mass,  from  the  burnt  cement  end 
round  to  the  raw  stone  end  ;  say  in  compartments  7  and  8 
to-day,  6  and  T  yesterday,  8  and  9  to-morrow,  advancing  one 
compartment  per  day,  like  the  drawing  and  setting. 

"  The  compartment  that  was  in  fire  yesterday,  say  No.  6,  is 
still  very  hot  to-day.  No.  5  less  hot.  No.  4  cooler,  and  so  on  to 
No.  2,  where  the  cement  is  cool  enough  to  be  handled,  and 
men  are  removing  it  from  the  kiln,  wlieelbarrows,  or  trucks 
on  portable  railway  tracks,  being  used  for  the  purpose. 


LIME-KILNS. 


37 


"  The  compartments  not  yet  fired  are  heated  by  the  hot 
gases  passing  through  them  to  the  chimney,  the  stone  in  the 
compartment  next  the  fire  being  at  a  full  red  heat,  while 
that  farthest  off,  which  was  put  in  yesterday,  is  only  warm. 

"  The  draught  of  the  chimney  is  sufiicient  to  draw  air  in  at 
the  open  doorways,  through  the  entire  mass  of  cement  and 
raw  stone,  to  the  open  flue,  which  is  the  one  by  the  cut-off. 

"  In  passing  through  the  burnt  cement  the  air  takes  up  the 
residue  of  heat  and  becomes  hotter  and  hotter,  till,  after  pas- 
sing through  the  cement  burned  yesterday,  the  hot  current 
ignites  at  once  the  dust  coal  as  it  falls  from  the  feed  pipes, 
and  the  gases  thus  formed  being  carried  on,  mixed  with  air, 
it  is  probable  the  stone  is  burned  considerably  in  advance  of 
w^here  the  coal  is  supplied. 

"  As  the  hot  gases  of  combustion  pass  on,  they  give  up  their 
heat  to  the  limestone,  till,  on  arriving  at  the  chimney,  there 
is  only  heat  enough  remaining  to  cause  a  draught  in  a  well- 
constructed  chimney  140  to  150  feet  in  height.  It  is  plain 
that  all  the  heat  of  combustion  is  utilized,  except  such  as  may 
escape  through  the  walls  of  the  kiln,  and  as  the  masonry  is 
very  massive,  the  loss  from  this  cause  is  very  slight. 

"  One  peculiar  feature  of  these  kilns  is,  that  although  less 
likely  to  get  out  of  order  than  other  kilns,  from  the  fact  that 
there  is  no  movement  in  the  burning  mass,  repairs  may  be 
easily  made  without  letting  the  fire  go  down. 

"  There  are  Hoffman  kilns  in  which  the  fires  have  not  been 
extinguished  for  five  years." 

78.  Methods  of  reducing-  the  calcined  stone  to  pow- 
der.— The  calcined  stone  may  be  reduced  to  powder,  either 
by  a  chemical  or  meclianical  process.  By  the  first,  water 
combines  with  the  lime,  forming  a  hydrate  of  lime,  which 
process  is  termed  slaking.  By  the  second  the  calcined 
stone  is  first  broken  into  small  lumps  ;  these  are  then  ground 
in  a  mill  to  the  requisite  degree  of  fineness,  ascertained  by 
the  sieves  through  which  the  ground  product  must  pass. 

79.  Slaking. — This  may  be  done  in  three  ways : 

By  pouring  sufiicient  water  on  the  burnt  stone  to  convert 
the  slaked  lime  into  a  thin  paste,  which  is  termed  drowning 
the  lime. 

By  placing  the  burnt  stone  in  a  basket,  and  immersing  it 
for  a  few  seconds  in  water,  during  which  time  it  will  imbibe 
enough  water  to  cause  it  to  fall,  by  slaking,  into  a  dry  pow- 
der ;  or  by  sprinkling  the  burnt  stone  with  a  sufiicient  quan- 
tity of  water  to  produce  the  same  effect. 

fey  allowing  the  stone  to  slake  spontaneously,  from  the 


38 


CIVIL  ENGINEEEING. 


moisture  it  imbibes  from  the  atmosphere,  which  is  termed 
air-slaJdng. 

80.  Opinion  seems  to  be  settled  among  engineers,  that 
drowning  is  the  worst  method  of  slaking  lime  which  is  to  be 
used  for  mortars.  "When  properly  done,  however,  it  produces 
a  finer  paste  than  either  of  the  other  methods ;  and  it  may 
therefore  be  resorted  to  whenever  a  paste  of  this  character,  or 
a  whitewash  is  wanted.  Some  care,  however,  is  requisite  to 
produce  this  result.  The  stone  should  be  fresh  from  the  kiln, 
otherwise  it  is  apt  to  slake  into  lumps  or  fine  grit.  All  the 
water  used  should  be  poured  over  the  stone  at  once,  which 
should  be  arranged  in  a  basin  or  vessel,  so  that  the  water  sur- 
rounding it  may  be  gradually  imbibed  as  the  slaking  proceeds. 
If  fresh  water  be  added  durino;  the  slaking,  it  checks  the 
process,  and  causes  a  gritty  paste  to  form. 

81.  In  slaking  by  immersion,  or  by  sprinkling  with  water, 
the  stone  should  be  reduced  to  small-sized  fraginents,  other- 
wise the  slaking  will  not  proceed  uniformly.  The  fat  limes 
should  be  in  lumps,  about  the  size  of  a  walnut,  for  immersion ; 
and,  when  withdrawn  from  the  water,  should  be  placed  im- 
mediately in  bins,  or  be  covered  with  sand,  to  confine  the 
heat  and  vapour.  If  left  exposed  to  the  air,  the  lime  becomes 
chilled  and  separates  into  a  coarse  grit,  which  takes  some  time 
to  slake  thoroughly  when  more  water  is  added.  Sprinkling 
the  lime  is  a  more  convenient  process  than  immersion,  and  is 
equally  good.  To  effect  the  slaking  in  this  way,  the  stone 
should  be  broken  into  fragments  of  a  suitable  size,  which  ex- 
periment will  determine,  and  be  placed  in  small  heaps,  sur- 
rounded by  sufticient  sand  to  cover  them  up  when  the  slaking 
is  nearly  completed.  The  stone  is  then  sj)rinkled  with  about 
one  fourth  its  bulk  of  water,  poured  through  the  rose  of  a 
watering-pot,  those  lumps  which  seem  to  slake  most  sluggishly 
receiving  the  most  water ;  when  the  process  seems  completed, 
the  heap  is  carefully  covered  over  with  the  sand,  and  allowed 
to  remain  a  day  or  two  before  it  is  used. 

82.  Slaking  either  by  immersion  or  by  sprinkling  is  con- 
sidered the  best.  The  quantity  of  water  imbibed  by  lime 
when  slaked  by  immersion,  varies  with  the  nature  of  the  lime  ; 
100  parts  of  fat  lime  will  take  up  only  18  parts  of  water ;  and. 
the  same  quantity  of  meager  lime  will  imbibe  from  20  to  35 
parts.  One  volume,  in  powder,  of  the  burnt  stone  of  rich  lime 
yields  from  1.50  to  1.70  in  vohime  of  powder  of  slaked  lime ; 
while  one  volume  of  meager  lime,  under  like  circumstances, 
will  yield  from  1.80  to  2.18  in  volume  of  slaked  lime. 

83.  Quick  lime,  when  exposed  to  the  free  action  of  the  air 


LIME.  39 


in  a  dry  locality,  slakes  slowly,  by  imbibing  moisture  from 
the  atmosphere,  with  a  slight  disengagement  of  heat.  Opinion 
seems  to  be  divided  with  regard  to  the  effect  of  this  method 
of  slaking  on  fat  limes.  Some  assert,  that  the  mortar  made 
from  them  is  better  than  that  obtained  from  any  other  process, 
and  attribute  this  result  to  the  re-conversion  of  a  portion  of 
the  slaked  lime  into  a  carbonate  ;  others  state  the  reverse  to 
obtain,  and  assign  the  same  canse  for  it.  With  regard  to 
hydraulic  limes,  all  agree  that  they  are  greatly  injured  by  air- 
slaking. 

84.  AVlien  the  slaking  is  imperfect  and  is  owing  as  in 
most  cases  to  the  stone  having  been  nnequally  burned,  the 
lime  should  be  reduced  to  a  paste  in  a  mortar  mill  that  will 
grind  fine  all  the  lumps.  This  is  particularly  necessary  in 
hydraulic  limes,  which  are  also  improved  in  energy  by  this 
reduction  of  the  nnderburned  lumps. 

85.  Air-slaked  fat  limes  increase  two-fifths  in  weight,  and 
for  one  volume  of  quick  lime  yield  3.52  volumes  of  slaked 
lime.    The  meager  limes  increase  one-eighth  in  weight,  and 
for  one  volume  of  quick  lime  yield  from  1.75  to  2.25  volumes  • 
of  slaked  lime. 

86.  The  dry  hydrates  of  lime,  when  exposed  to  the  at- 
mosphere, gradually  absorb  carbonic  acid  and  water.  This 
process  proceeds  very  slowly,  and  the  slaked  lime  never  re- 
gains all  the  carbonic  acid  which  is  driven  off  by  the  calcina- 
tion of  the  lime-stone.  When  converted  into  a  thick  paste, 
and  exposed  to  the  air,  the  hydrates  gradually  absorb  carbonic 
acid ;  this  action  first  takes  place  on  the  surface,  and  proceeds 
more  slowly  from  year  to  year  towards  the  interior  of  the  ex- 
posed mass.  The  absorption  of  gas  proceeds  more  rapidly  in 
the  meager  than  in  the  fat  limes.  Those  hydrates  which  are 
most  thoroughly  slaked  become  hardest.  The  hydrates  of  the 
pure  fat  limes  become  in  time  very  hard,  while  those  of  the 
hydraulic  limes  become  only  moderately  hard. 

87.  The  fat  limes,  when  slaked  by  drowning,  may  be  pre- 
served for  a  long  period  in  the  state  of  paste,  if  placed  in  a 
damp  situation  and  kept  from  contact  with  the  air.  They 
may  also  be  preserved  for  a  long  time  without  change,  when 
slaked  by  immersion  to  a  dry  powder,  if  placed  in  covered 
vessels.  Hydraulic  limes,  under  similar  circumstances,  will 
harden  if  kept  in  the  state  of  paste,  and  will  deteriorate  when 
in  powder,  unless  kept  in  perfectly  air-tight  vessels. 

88.  The  hydrates  of  fat  lime,  from  air-slaking  or  immersion, 
require  a  smaller  quantity  of  water  to  reduce  them  to  the  state 
of  paste  than  the  others ;  but,  when  immersed  in  water,  they 


40 


CIVIL  ENGn^EEEING. 


gradually  imbibe  their  full  dose  of  water,  the  paste  becom- 
ing thicker,  but  remaining  unchanged  in  volume.  Exposed 
in  this  way,  the  water  will  in  time  dissolve  out  all  the  lime  of 
the  hydrate  which  has  not  been  reconverted  into  a  sub-carbo- 
nate, by  the  absorption  of  carbonic  acid  before  immersion ; 
and  if  the  water  contain  carbonic  acid,  it  will  also  dissolve  the 
carbonated  portions. 

89.  The  hydrates  of  hydraulic  lime,  when  immersed  in 
water  in  the  state  of  thin  pastes,  reject  a  portion  of  the  water 
from  the  paste,  and  become  hard  in  time  ;  if  the  paste  be 
very  stiff,  they  imbibe  more  water,  set  quickly,  and  acquire 
greater  hardness  in  time  than  the  soft  pastes.  The  pastes  of 
the  hydrates  of  hydraulic  lime,  which  have  hardened  in  the 
air,  will  retain  their  hardness  when  placed  in  water. 

90.  All  limes  seem  to  have  their  hydraulic  energy  affected 
by  the  degree  of  their  calcination  ;  but  only  in  their  first 
stages  of  immersion.  This  is  observed  even  in  underburned 
common  lime  which,  when  suitably  reduced,  is  found  to  be 
slightly  hydraulic. 

91.  The  pastes  of  the  fat  limes  shrink  very  unequally  in 
drying,  and  the  shrinkage  increases  with  the  purity  of  the 
lime ;  on  this  account  it  is  difficult  to  apply  them  alone  to  any 
building  purposes,  except  in  very  thin  layers.  The  pastes  of 
the  hydraulic  limes  can  only  be  used  with  advantage  nnder 
water,  or  where  they  are  constantly  exposed  to  humidity ;  and 
in  these  situations  they  are  never  used  alone,  as  they  are 
found  to  succeed  as  well,  and  to  present  more  economy,  when 
mixed  with  a  portion  of  sand. 

92.  Manner  of  redueing  hydraulic  cement. — As  the 
cement  stones  will  not  slake,  they  must  be  reduced  to  a  fine 
powder  by  some  mechanical  process,  before  they  can  be  con 
verted  into  a  hydrate.  They  methods  usually  employed  for 
this  purpose  consist  in  first  breaking  the  burnt  stone  into  small 
fragments,  either  under  iron  cylinders,  or  in  conical-shaped 
mills  suitably  formed  for  this  purpose.  The  product  is  next 
ground  between  a  pair  of  stones,  or  else  crushed  by  an  iron 
roller.  The  coarser  particles  are  separated  from  the  fine 
powder  by  the  ordinary  processes  with  sieves.  The  powder 
is  then  carefully  packed  in  air-tight  casks,  and  kept  tor  use. 

93.  Hydraulic  cement,  like  hydraulic  lime,  deteriorates  by 
exposure  to  the  air,  and  may  in  time  lose  all  its  hydraulic 
properties.  On  this  account  it  should  be  used  when  fresh 
irom  the  kiln  ;  for,  however  carefully  packed,  it  cannot  be 
well  preserved  when  transported  to  any  distance. 

94.  The  deterioration  of  hydraulic  cements,  from  exposure 


LIME. 


41 


to  the  air,  arises,  probably,  from  a  cbemical  disunion  between 
the  constituent  elements  of  the  burnt  stone,  occasioned  by 
the  absorption  of  water  and  carbonic  acid.  AVhen  injured, 
their  energy  can  be  restored  by  submitting  them  to  a  much 
slighter  degree  of  heat  than  that  which  is  requisite  to  calcine 
the  stone  suitably  in  the  first  instance.  From  the  experi- 
ments of  M.  Petot,  it  appears  that  a  red  heat,  kept  up  for 
a  short  period,  is  sufficient  to  restore  damaged  hydraulic 
cements. 

95.  "  As  a  rule,  all  hydraulic  cements  produced  at  a  low 
heat,  whether  derived  from  argillaceous  or  argillo-magnesian 
lime-stones,  are  light  in  weight  and  quick-setting,  and  never 
attain,  when  made  into  mortar  or  beton,  more  than  30  to  33 
per  cent,  of  the  strength  and  hardness  of  Portland  cement 
placed  in  similar  circumstances.  They  are  also  greatly  in- 
ferior to  good  hydraulic  lime.  This  is  true  of  all  cements 
made  at  a  low  heat,  including  even  those  derived  from  lime- 
stones, that  might,  with  proper  burning,  have  yielded  Portland 
cement.  The  celebrated  Roman  cement,  the  twice-kilned 
artificial  cements,  the  quick-setting  French  cement,  like  that 
of  Yassy,  and  all  the  hydraulic  cements  manufactured  at  the 
present  day  in  the  United  States,  belong  to  this  category." 

96.  ARTIFICIAL  HYDRAULIC  LIMES  AND  CE- 
MENTS.- The  discovery  of  the  argillaceous  character  of  the 
stones  which  yield  hydraulic  limes  and  cements,  connected 
with  the  fact  that  brick  reduced  to  a  fine  powder,  as  well  as 
several  substances  of  volcanic  origin  having  nearly  the  same 
constituent  elements  as  ordinary  brick,  when  mixed  in  suita- 
ble proportions  with  common  lime,  will  yield  a  paste  that 
hardens  under  water,  has  led,  within  a  recent  period,  to  arti- 
ficial methods  of  producing  compounds  possessing  the  proper- 
ties of  natural  hydraulic  limestones. 

97.  M.  Yicat  was  the  first  to  point  out  the  method  of  form- 
ing an  artificial  hydraulic  lime,  by  mixing  common  lime  and 
unburnt  clay,  in  suitable  proportions,  and  then  calcining 
them.  The  experiments  of  M.  Yicat  have  been  repeated  by 
several  eminent  engineers  with  complete  success,  and  among 
others  by  General  Pasley,  who,  in  a  recent  work  by  him, 
Observations  on  Limes,  Calcareous  Cements,  etc.,  has  given, 
with  minute  detail,  the  results  of  his  experiments ;  from  which 
it  appears  that  an  hydraulic  cement,  fully  equal  in  quality  to 
that  obtained  from  natural  stones,  can  be  made  by  mixing 
common  lime,  either  in  the  state  of  a  carbonate  or  of  a  hy- 
drate, with  clay,  and  subjecting  the  mixture  to  a  suitable  de- 


» 


42  CIVIL  ENGINEEEING. 

gree  of  heat.  In  some  parts  of  France,  where  chalk  is  found 
abundantly,  the  preparation  of  artificial  hydraulic  lime  has 
become  a  branch  of  manufacture. 

98.  Different  methods  have  been  pursued  in  ;preparing  this 
material,  the  main  object  being  to  secure  the  Imest  mechan- 
ical division  of  the  two  ingredients,  and  their  thorough  mix- 
ture. For  this  purpose  the  lime-stone,  if  soft,  like  (^iialk  or 
tufa,  may  be  reduced  in  a  wash-mill,  or  a  rolling-mill,  to  the 
state  of  a  soft  pulp  ;  it  is  then  incorporated  with  the  clay,  by 
passing  them  through  a  pug-mill.  The  mixture  is  next 
moulded  into  small  blocks,  or  made  up  into  balls  between  2 
and  3  inches  diameter,  by  hand,  and  well  dried.  The  balls 
are  placed  in  a  kiln, — suitably  calcined,  and  are  finally  slaked, 
or  ground  down  fine  for  use. 

99.  If  the  lime-stone  be  hard,  it  must  be  calcined  and 
slaked  in  the  usual  manner,  before  it  can  be  mixed  with  the 
clay.  The  process  for  mixing  the  ingredients,  their  calcina- 
tion, and  further  preparation  for  use,  are  the  same  as  in  the 
preceding  case. 

100.  The  artificial  hydraulic  cement  manufactured  in 
France,  at  Boulogne,  and  possessing  the  same  qualities  as  the 
artificial  Portland  cement,  is  composed  of  79.5  per  cent,  of 
carbonate  of  lime  in  powder,  and  20.5  of  clay,  which,  after 
being  thoroughly  mixed,  are  subjected  to  a  very  high  degree 
of  temperature. 

101.  What  is  known,  in  commerce -and  among  engineers, 
as  artificial  Portland  cement,  is  a  mixture  of  tlie  blue  clay  of 
the  London  basin  and  chalk,  formed  by  grinding  the  materials 
together  in  water.  The  semi-fluid  mixture  is  run  off  into 
vats,  and,  after  settling  and  attaining  sufficient  consistency,  is 
dried  by  artificial  heat  and  then  calcined,  at  a  high  temj^era- 
ture,  to  the  verge  of  vitrification.  It  is  then  reduced  for  use 
to  a  very  fine  powder.  It  is  said  not  to  deteriorate  from  ex- 
posure to  the  air,  provided  it  be  kept  from  moisture. 

102.  Artificial  hydraulic  lime,  prepared  from  the  hard 
limestones,  is  more  expensive  than  that  made  from  the  soft ; 
but  it  is  stated  to  be  superior  in  quality  to  the  latter. 

103.  As  clays  are  seldom  free  from  carbonate  of  lime,  and 
as  the  limestones  which  yield  common  or  fat  lime  may  con- 
tain some  portion  of  clay,  the  proper  proportic^ns  of  the  two 
ingredients,  to  produce  either  an  hydraulic  lime  or  a  cement, 
must  be  determined  by  experiment  in  each  case,  guided  by  a 
previous  analysis  of  the  two  ingredients  to  be  tried. 

If  the  lime  be  pure,  and  the  clay  be  free  from  lime,  then 
the  combinations  in  the  proportions  given  in  the  table  of  M. 


LIME. 


43 


Petot  will  give,  by  calcination,  like  results  with  the  same 
proportions  when  found  naturally  combined. 

104.  Puzzolana,  etc.  The  pi-actice  of  using  brick  or  tile- 
dust,  or  a  Tolcanic  substance  known  by  the  name  of  puzzo- 
lana, mixed  with  common  lime,  to  form  an  hydraulic  lime, 
was  known  to  the  Romans,  by  whom  mortars  composed  of  these 
materials  were  extensively  used  in  their  h3^draulic  constructions. 
This  practice  has  been  more  or  less  followed  by  modern  engi- 
neers, who,  until  within  a  few  years,  either  used  the  puzzolana 
of  Italy,  where  it  is  obtained  near  Mount  Yesuvius,  in  a  pul- 
yeriilent  state,  or  a  material  termed  Trass,  manufactured  in 
Holland,  by  grinding  to  a  fine  powder  a  volcanic  stone  obtained 
near  Andernach,  on  the  Rhine. 

Experiments  by  several  eminent  chemists  have  extended 
the  list  of  natural  substances  which,  when  properly  burnt  and 
reduced  to  powder,  have  the  same  properties  as  puzzolana-. 
They  mostly  belong  to  the  feldspathic  and  schistose  rocks, 
and  are  either  fine  sand,  or  clays  more  or  less  indurated. 

The  following  Table  gives  the  results  of  analyses  of  Puzzo- 
lana, Trass,  a  Basalt,  and  a  Schistus,  which,  when  hurnt 
and  powdered,  were  found  to  possess  the  properties  of 
puzzolana. 


Puzzolana. 

Trass. 

Basalt. 

Schistus. 

0.445 

0.570 

44.50 

46.00 

Alumina  

0.150 

0.120 

16.75 

26.00 

Lime  

0.088 

0.026 

9.50 

4.00 

Magnesia  

0.047 

0.010 

0.120 

0.050 

20.00 

14.00 

Oxide  of  manganese  

2.37 

8.00 

0.014 

0.070 

0.030 

0.010 

2.60 

0.106 

0.144 

4.28 

2.00 

1.000 

1.000 

looToo" 

100.00 

105.  Wliether  natural  puzzolanas  occur  in  the  United 
States,  is  not  known.  The  great  abundance  of  natural  hy- 
draulic cements  would  probably  cause  no  demand  for  them, 
nor  for  artificial  puzzolanas  for  building  purposes. 

106.  All  of  these  substances,  when  prepared  artificially, 
are  now  generally  known  by  the  name  of  arttficial puzzolanas, 
in  contradistinction  to  those  wdiich  occur  naturally. 

107.  General  Treussart,  of  the  French  Corps  of  Military 
Engineers,  first  attem^pted  a  systematic  investigation  of  the 


44 


CIVIL  ENGINEERING. 


properties  of  artificial  piizzolanas  made  from  ordinary  clay, 
and  of  the  best  manner  of  preparing  them  on  a  large  scale. 
It  appears  from  the  results  of  his  experiments,  that  the  plas- 
tic clays  used  for  tiles,  or  pottery,  which  are  unctuous  to  the 
touch,  the  alumina  in  them  being  in  the  proportion  of  one 
fiftli  to  one  third  of  the  silica,  furnish  the  best  artificial  puzzo- 
lanas  when  suitably  burned.  The  clays  which  are  more  mea- 
ger, and  harsher  to  the  touch,  yield  an  inferior  article,  but  are 
in  some  cases  preferable,  from  the  greater  ease  with  which 
they  can  be  reduced  to  a  powder. 

108.  As  the  clays  mostly  contain  lime,  magnesia,  some  of 
the  metallic  oxides,  and  alkaline  salts.  General  Treussart  en- 
deavored to  ascertain  the  influence  of  these  substances  upon 
the  qualities  of  the  artificial  puzzolanas  from  clays  in  which 
they  are  found.  He  states,  that  the  carbonate  of  potash  and 
the  muriate  of  soda  seem  to  act  beneficially ;  that  magnesia 
seems  to  be  passive,  as  well  as  the  oxide  of  iron,  except  when 
the  latter  is  found  in  a  large  proportion,  when  it  acts  hurtful- 
ly ;  and  that  the  lime  has  a  material  influence  on  the  degree 
of  heat  required  to  convert  the  clay  into  a  good  artificial  puz- 
zolana. 

109.  The  management  of  the  heat,  in  the  preparation  of 
this  material,  seems  of  the  first  consequence ;  and  General 
Treussart  recommends  that  direct  experiment  be  resorted  to, 
as  the  most  certain  means  of  ascertaining  the  proper  point. 
For  this  purpose,  specimens  of  the  clay  to  be  tried  may  be 
kneaded  into  balls  as  large  as  an  egg,  and  the  balls  when  dry, 
be  submitted  to  different  degrees  of  heat  in  a  kiln,  or  furnace, 
through  which  a  current  of  air  must  pass  over  the  balls,  as 
this  last  circumstance  is  essential  to  secure  a  material  possess- 
ing the  best  hydraulic  qualities.  Some  of  the  balls  are  with- 
drawn as  soon  as  their  color  indicates  that  they  are  under- 
burnt  ;  others  when  they  have  the  appearance  of  well-burnt 
brick ;  and  others  when  their  color  shows  that  they  are  over- 
burnt,  but  before  they  become  vitrified.  The  burnt  balls  are 
reduced  to  an  impalpable  powder,  and  this  is  mixed  with  a 
hydrate  of  fat  lime,  in  the  proportion  of  two  parts  of  the  pow- 
der to  one  of  lime  in  paste.  Water  is  added,  if  necessary,  to 
bring  the  different  mixtures  to  the  consistence  of  a  thick  pulp ; 
and  they  are  separately  placed  in  glass  vessels,  covered  with 
water,  and  allowed  to  remain  until  the}^  harden.  The  com- 
pound which  hardens  most  promptly  w411  indicate  the  most 
suitable  degree  of  heat  to  be  applied. 

110.  As  the  carbonates  of  lime,  of  potash,  and  of  soda,  act 
as  fluxes  on  silica,  the  presence  of  either  one  of  them  will 


LIME. 


45 


modify  the  degree  of  heat  necessary  to  convert  the  clay  into 
a  good  natural  puzzolana.  Clay,  containing  about  one  tenth 
of  lime,  should  be  brought  to  about  the  state  of  slightly-burnt 
brick.  The  ochreous  clays  require  a  higher  degree  of  heat  to 
convert  them  into  a  good  material,  and  should  be  burnt  until 
they  assume  the  appearance  of  well-burnt  brick.  The  more 
refractory  clays  will  bear  a  still  higher  degree  of  heat ;  but 
the  calcination  should  in  no  case  5e  carried  to  the  point  of 
incipient  vitrification. 

111.  The  quantity  of  lime  contained  in  the  clay  can  be  read- 
ily ascertained  beforehand,  by  treating  a  small  portion  of  the 
clay,  diffused  in  water,  with  enough  muriatic  acid  to  dissolve 
out  the  lime ;  and  this  last  might  serve  as  a  guide  in  the  pre- 
liminary stages  of  the  experiments. 

112.  General  Treussart  states,  as  the  results  of  his  experi- 
ments, that  the  mixture  of  artificial  puzzolana  and  fat  lime 
forms  an  hydraulic  paste  superior  in  quality  to  that  obtained 
by  M.  Yicat's  process  for  making  artificial  hydraulic  lime. 
M.  Curtois,  a  French  civil  engineer,  in  a  memoir  on  these  ar- 
tificial compounds,  published  in  the  Annales  des  Ponts  et 
Chaussees,  1834,  and  General  Pasley,  more  recently,  adopt 
the  conclusion  of  General  Treussart.  M.  Yicat's  process  ap- 
pears best  adapted  when  chalk,  or  any  very  soft  lime-stone, 
which  can  be  readily  converted  to  a  soft  pulp,  is  used,  as 
offering  more  economy,  and  affording  an  hydraulic  lime  which 
is  sufficiently  strong  for  most  building  purposes.  By  it  Gen- 
eral Pasley  has  succeeded  in  obtaining  an  artificial  hydraulic 
cement  which  is  but  little,  if  at  all,  inferior  to  the  best  natu- 
ral varieties ;  a  result  which  has  not  been  obtained  from  any 
combination  of  fat  lime  with  puzzolana,  whether  natural  or 
artificial. 

113.  All  the  puzzolanas  possess  the  important  property  of 
not  deteriorating  by  exposure  to  the  air,  which  is  not  the  case 
with  any  of  the  hydraulic  limes  or  cements.  This  property 
may  render  them  very  serviceable  in  many  localities,  where 
only  common  or  feebly  hydraulic  lime  can  be  obtained. 

114.  The  well-known  artificial  Portland  cement,  manufac- 
tured in  England,  is  composed  of  an  intimate  mixture  of  chalk 
and  clay,  in  the  state  of  paste,  which  is  then  dried  and  burned 
in  kilns  or  ovens ;  the  product  of  the  calcination  being  fiinty, 
or  like  vitrified  brick.  This  degree  of  calcination  is  essential 
to  the  excellence  of  the  material,  of  which  its  weight,  or  spe- 
cific gravity,  is  one  of  the  best  tests. 

Another  more  recent  method  of  giving  a  certain  degree  of 
hydraulicity  to  common  limes,  and  of  improving  that  of  hy- 


46 


CIVIL  ENGINEEKllfG. 


draulic  limes,  is  to  place  the  calcined  stone,  after  it  has  been 
drawn  from  the  kihi,  in  arched  ovens  which  can  be  made  air- 
tight, in  which  it  can  be  subjected  to  the  action  of  a  fire,  from 
a  grate  beneath;  so  that  the  heat  can  be  equally  diffused 
throughout  the  mass,  which  is  brought  only  to  a  slight  glow, 
as  seen  by  the  eye.  When  in  this  condition,  iron  pots  contain- 
ing sulphur  are  placed  underneath,  and  the  sulphur,  converted 
into  vapour,  allowed  to  permeate  the  mass  of  lime  ;  the  escape 
of  the  vapour  from  the  oven  having  been  previously  provided 
against.  After  the  sul^Dhur  has  been  consumed  the  mass  is 
allowed  to  cool,  and  is  then  ground  fine  like  other  cements. 
This  product  is  known  in  commerce  as  Scotfs  cement,  from 
the  name  of  th<s  inventor,  an  ofiicer  of  the  Koyal  Engineers. 
See  Professional  Pct^ers  of  ike  Corjps  of  lioijal  Engineers. 
Vol.  X.    New  Series. 


lY. 

MORTAR. 

115.  Mortar  is  any  mixture  of  lime  in  paste  with  sand.  It 
may  be  divided  into  two  principal  classes ;  Hydraulic  mor- 
tar, which  is  made  of  hydraulic  lime,  and  Common  mortar^ 
made  of  common  lime. 

316.  The  term  Grout  is  applied  to  any  mortar  in  a  thin  or 
fluid  state  ;  and  the  terms  Concrete  and  Beton,  to  mortars  in- 
corporated witli  gravel  and  small  fragments  of  stone  or  brick. 

117.  Mortar  is  used  for  various  purposes  in  building.  It 
serves  as  a  cement  to  unite  blocks  of  stone,  or  brick.  In  con- 
crete and  beton,  which  may  be  regarded  as  artificial  conglom- 
erate stones,  it  forms  the  matrix  by  w^hich  the  gravel  and 
broken  stone  are  held  together ;  and  it  is  the  principal  mate- 
rial with  which  the  exterior  surfaces  of  walls  and  the  interior 
of  edifices  are  coated. 

118.  The  quality  of  mortars,  whether  used  for  structures 
exposed  to  the  weather,  or  for  those  immersed  in  water,  will 
depend  upon  the  nature  of  the  materials  used  ;  their  propor- 
tions ;  the  manner  in  which  the  lime  has  been  converted  in- 
to a  paste  to  receive  the  sand  ;  and  the  mode  employed  to 
mix  the  ingredients.    Upon  all  of  these  points  experiment 


MOETAK. 


47 


is  the  only  unerring  guide  for  the  engineer  ;  for  the 
great  diversity  in  the  constituent  elements  of  limestones,  as 
Avell  as  in  the  other  ingredients  of  mortars,  must  necessarily 
alone  give  rise  to  diversities  in  results  ;  and  when,  to  these 
causes  of  variation,  are  superadded  those  resulting  from  dif- 
ferent processes  pursued  in  the  manipulations  of  slaking  the 
lime  and  mixing  the  ingredients,  no  surprise  should  be  felt  at 
the  seemingly  opposite  conclusions  at  which  w^riters,  who  have 
pursued  the  subject  experimentally,  have  arrived.  From  the 
great  mass  of  facts,  however,  presented  on  this  subject  within 
a  few  years,  some  general  rules  may  be  laid  down,  which  the 
engineer  may  safely  follow,  in  the  absence  of  the  means  of 
making  direct  experiments. 

119.  As  to  the  action  of  salt  water  on  artificial  hydraulic 
limes  made  by  mixing  common  lime  with  a  natural  or  artifi- 
cial puzzolana,  opinion  among  European  engineers  seems  di- 
vided. Some  state  that  they  withstand  well  the  action  of  salt 
water  ;  others  that  they  only  resist  this  action  after  the  expos- 
ed surface  becomes  coated  with  barnacles,  oysters,  etc. 

120.  The  view  now  generally  taken  of  mortar  is,  that  being 
an  artificial  sandstone,  the  nearer  its  constituents  approach  ♦ 
those  of  the  natural  sandstones,  the  better  will  be  the  result ., 
obtained  ;  and  that  therefore  the  best  proportions  for  its  in- 
gredients are  those  in  which  each  grain  of  sand  is  enveloped 
with  just  sufiicient  lime,  in  a  barely  moist  state,  to  cause  the 
whole  mass  to  cohere  and  set  quickly.  Too  much  lime  causes 
shrinkage  and  cracks  ;  and  w^hen  too  much  water  is  added 
the  mass  in  drying  is  found  to  be  porous. 

121.  Sand.  This  material,  which  forms  one  of  the  ingre- 
dients of  mortar,  is  the  granular  product  arising  from  the  dis- 
integration of  rocks.  It  may,  therefore,  like  the  rocks  from 
which  it  is  derived,  be  divided  into  three  principal  varieties 
— the  silicious,  the  calcareous,  and  the  argillaceous. 

Sand  is  also  named  from  the  locality  where  it  is  obtained, 
pit  sand,  which  is  procured  from  excavations  in  alluvial,  or 
other  deposits  of  disintegrated  rock ;  river  sand,  and  sea  sand, 
which  are  taken  from  the  shores  of  the  sea,  or  rivers. 

Builders  again  classify  sand  according  to  the  size  of  the 
grain.  The  term  coarse  sand  is  applied  when  the  grain  va- 
ries between  |-th  and  yV^^  inch  in  diameter ;  the  term^;^^ 
sand,  wdien  the  grain  is  between  -Y^ih.  and  -g-V^h  of  an  inch  in 
diameter  ;  and  the  term  mixed  sand  is  used  for  any  mixture 
of  the  two  preceding  kinds. 

122.  The  silicious  sands,  arising  from  the  quartzose  rocks, 
are  the  most  abundant,  and  are  usually  preferred  by  builders. 


48 


CIVrL  ENGINEEEING. 


The  calcareous  sands,  from  hard  calcareous  rocks,  are  more 
rare,  but  form  a  good  ingredient  for  mortar.  Some  of  the 
argillaceous  sands  possess  the  properties  of  the  less  energetic 
puzzolanas,  and  are  therefore  very  valuable,  as  forming  with 
common  lime  an  artificial  hydraulic  lime. 

123.  The  property  which  some  argillaceous  sands  possess, 
of  forming  with  common,  or  slightly  hydraulic  lime  a  com- 
pound which  will  harden  under  water,  has  been  long  known 
in  France,  where  these  sands  are  termed  arenes.  The  sands 
of  this  nature  are  usually  found  in  hillocks  along  river  valleys. 
These  hillocks  sometimes  rest  on  calcareous  rocks,  or  argil- 
laceous tufas,  and  are  frequently  formed  of  alternate  beds  of 
the  sand  and  pebbles.  The  sand  is  of  various  colors,  such  as 
yellow,  red,  and  green,  and  seems  to  have  been  formed  from 
the  disintegration  of  clay  in  a  more  or  less  indurated  state. 
The  arenes  are  not  as  energetic  as  either  natural  or  artificial 
puzzolanas  ;  still  they  form,  with  common  lime,  an  excellent 
mortar  for  masonry  exposed  either  to  the  open  air,  or  to 
humid  localities,  as  the  foundations  of  edifices. 

124.  Pit-sand  has  a  rougher  and  more  angular  grain  than 
river  or  sea  sand  ;  and,  on  this  account,  is  generally  prefer- 
red by  builders  for  mortars  used  for  brick,  or  stone-work. 
Whether  it  forms  a  stronger  mortar  than  the  other  two  is  not 
positively  settled,  although  some  experimejits  would  lead  to 
the  conclusion  that  it  does. 

125.  River  and  sea  sand  are  by  some  preferred  for  plaster- 
ing, because  they  are  whiter,  and  have  a  finer  and  more  uni- 
form grain  than  pit  sand  ;  but  as  the  sands  from  the  shores  of 
tidal  waters  contain  salts,  they  should  not  be  used,  owing  to 
their  hygrometric  properties,  before  the  salts  are  dissolved  out 
in  fresh  water  by  careful  washing. 

126.  Pit  sand  is  seldom  obtained  free  from  a  mixture  of 
dirt,  or  clay  ;  and  these,  when  found  in  any  notable  quantity 
in  it,  give  a  weak  and  bad  mortar.  Earthy  sands  should, 
therefore,  be  cleansed  from  dirt  before  using  them  for  mor- 
tar ;  this  may  be  effected  by  washing  the  sand  in  shallow  vats, 
and  allowing  the  turbid  water,  in  which  the  clay,  dust  and 
other  like  impurities  are  held  in  suspension,  to  run  off. 

127.  Sand,  when  pure  or  well  cleansed,  may  be  known  by 
not  soilino^  the  finojers  when  rubbed  between  them. 

128.  Hydraulic  mortar-  This  material  may  be  made 
from  the  natural  hydraulic  limes  ;  from  those  which  are  pre- 
pared by  M.  Yicat's  process  ;  or  from  a  mixture  of  common 
or  feebly  hydraulic  lime  with  a  natural  or  artificial  puzzolana. 
All  writers,  however,  agree  that  it  is  better  to  use  a  natural 


MORTAE. 


49 


than  an  artificial  hydraulic  lime,  when  the  former  can  be 
readily  procured. 

129.  AVTien  the  lime  used  is  strongly  hydraulic,  M.  Yicat  is 
of  opinion  that  sand  alone  should  be  used  with  it,  to  form 
a  good  hydraulic  mortar.  General  Treussart  has  drawn  the 
conclusion,  from  his  experiments,  that  the  mortar  of  all  hy- 
draulic limes  is  improved  by  an  addition  of  a  natural  or  arti- 
ficial puzzolana.  The  quantity  of  sand  used  may  vary  from 
IJ  to  2  parts  of  the  lime  in  bulk,  when  reduced  to  a  thick 
pulp. 

130.  The  practice  of  the  United  States  Corps  of  Engineers, 
in  the  construction  of  heavy  masonry,  has  been  to  add  from 
2.5  to  3.5,  in  bulk,  of  compact  sand  to  one  of  lime  of  a  thick 
paste  in  the  composition  of  their  hydraulic  mortars  ;  and  it 
has  been  found  that  an  equal  bulk  of  common  lime  in  paste 
can  be  mixed  with  hydraulic  cement  paste  without  occasion- 
ing any  material  diminution  in  the  strength  of  the  resulting 
mortar. 

131.  For  hydraulic  mortars,  made  of  common,  feeble,  or  or- 
dinary hydraulic  limes,  and  artificial  puzzolana,  M.  Yicat 
states  that  the  puzzolana  should  be  the  weaker  as  the  lime  is 
more  strongly  hydraulic ;  using,  for  example,  a  very  ener- 
getic puzzolana  with  a  fat  or  a  feebly  hydraulic  lime.  The 
proportion  of  sand  which  can  be  incorporated  with  these  in- 
gredients, to  form  an  hydraulic  mortar,  is  stated  by  General 
Treussart  to  be  one  volume  to  one  of  puzzolana,  and  one  of 
lime  in  paste. 

132.  In  proportioning  the  ingredients,  the  object  to  which 
the  mortar  is  to  be  applied  should  be  regarded.  When  it  is 
to  serve  to  unite  stone,  or  brick  work,  it  is  better  that  the  hy- 
draulic lime  should  be  rather  in  excess :  when  it  is  used  as  a 
matrix  for  beton,  no  more  lime  should  be  used  than  is  strictly 
required.  In"o  harm  will  arise  from  an  excess  of  good  hydrau- 
lic lime,  in  any  case ;  but  an  excess  of  common  lime  is  injuri- 
ous to  the  quality  of  the  mortar. 

133.  Common  and  ordinary  hydraulic  limes,  when  made 
into  mortar  with  arenes,  give  a  good  material  for  hydraulic 
purposes.  The  proportions  in  which  these  have  been  found 
to  succeed  well,  are  one  of  lime  to  three  of  arenes. 

134.  Hydraulic  cement,  from  the  promptitude  with  which 
it  hardens,  both  in  the  air  and  under  water,  is  an  invalu- 
able material  where  this  property  is  essential.  Any  dose  of 
sand  injures  its  properties  as  a  cement.  But  hydraulic  ce- 
ment may  be  added  with  decided  advantage  to  a  mortar  of 
common,  or  of  feebly  hydraulic  lime  and  sand.    It  is  in  this 

4 


50 


CIVIL  ENGINEERING. 


way  that  it  is  generally  used  in  our  public  works.  The  French 
engineers  give  the  preference  to  a  good  hydraulic  mortar  over 
hydraulic  cement,  both  for  uniting  stone,  or  brick  work,  and 
for  plastering.  They  find,  from  their  practice,  that  when 
used  as  a  stucco,  it  does  not  withstand  well  the  effects  of 
weather  ;  that  it  swells  and  cracks  in  time ;  and,  when  laid  on 
in  successive  coats,  that  they  become  detached  from  each 
other. 

General  Pasley,  who  has  paid  great  attention  to  the  pro- 
perties of  natural  and  artificial  hydraulic  cements,  does  not 
agree  with  the  French  engineers  in  his  conclusions.  He  states 
that,  when  skilfully  applied,  hydraulic  cement  is  superior  to 
any  hydraulic  mortar  for  masonry,  but  that  it  must  be  used 
only  in  thin  joints,  and  when  applied  as  a  stucco,  that  it 
should  be  laid  on  in  but  one  coat ;  or,  if  it  be  laid  on  in  two, 
the  second  nmst  be  added  long  before  the  first  has  set,  so  that, 
in  fact,  the  two  make  but  one  coat.  By  attending  to  these 
precautions.  General  Pasley  states  that  a  stucco  of  hydraulic 
cement  and  sand  will  withstand  perfectly  the  effects  of  frost. 

135.  Mortars  exposed  to  weather. — The  French  engi- 
neers, who  have  paid  great  attention  to  the  subject  of  mortars, 
coincide  in  the  opinion,  that  a  mortar  cannot  be  made  of  fat 
lime  and  any  inert  sands,  like  those  of  the  silicious,  or  calca- 
reous kinds,  which  will  withstand  the  ordinary  exposure  of 
weather  ;  and  that,  to  obtain  a  good  mortar  for  this  purpose, 
either  the  hydraulic  limes  mixed  with  sand  must  be  employed, 
or  else  common  lime  mixed  either  with  arhies,  or  with  a  puz- 
zolana  and  sand. 

136.  Any  pure  sand,  mixed  in  proper  proportions  with  hy- 
draulic lime,  will  give  a  good  mortar  for  the  open  air ;  but 
the  hardness  of  the  mortar  will  be  affected  by  the  size  of  the 
grain,  particularly  when  hydraulic  lime  is  used.  Fine  sand 
yields  the  best  mortar  with  good  hydraulic  lime ;  mixed  sand 
with  the  feebly  hydraulic  limes ;  and  coarse  sand  with  fat 
lime. 

137.  For  mortar  to  be  used  for  filling  the  exterior  of  the 
joints,  or  as  it  is  termed,  for  pointing,  the  amount  of  lime  paste 
m  bulk  should  be  but  slightly  greater  than  that  of  the  void 
spaces  of  grains  of  sand.  The  bulk  of  sand  for  this  purpose 
should  be  from  2.5  to  2.75  that  of  the  lime  paste. 

133.  The  proportion  which  the  lime  should  bear  to  the 
sand  seems  to  de^^end,  in  some  measure,  on  the  manner  in 
which  the  lime  is  slaked.  M.  Vicat  states,  that  the  strength 
of  mortar  made  of  a  stiff  paste  of  fat  lime,  slaked  in  the  ordi- 
nary way,  increases  from  0.50  to  2.40  to  one  of  the  paste  in 


MORTAR. 


61 


volume ;  and  that,  wiien  the  lime  is  slaked  bj  immersion,  one 
volume  of  the  like  paste  will  give  a  mortar  that  increases  in 
strength  from  0.50  to  2.20  parts  of  sand. 

For  one  volume  of  a  paste  of  hydraulic  lime,  slaked  in  the 
ordinary  way,  the  strength  of  the  mortar  increases  from  0  to 
1.80  parts  of  sand ;  and,  when  slaked  by  immersion,  the  mor- 
tar of  a  like  paste  increases  in  strength  from  0  to  1.70  parts 
of  sand.  In  every  case,  when  the  dose  of  sand  was  increased 
beyond  these  proportions,  the  strength  of  the  resulting  mortar 
was  found  to  decrease. 

139.  Manipulations  of  mortar. — The  quality  of  hydrau- 
lic mortar,  which  is  to  be  immersed  in  w^ater,  is  more  affected 
by  the  manner  in  which  the  lime  is  slaked,  and  the  ingredients 
mixed,  than  that  of  mortar  which  is  to  be  exposed  to  the 
weather ;  although  in  both  cases  the  increase  of  strength,  by 
the  best  manipulations,  is  sufficient  to  make  a  study  of  them 
a  matter  of  some  consequence. 

140.  The  results  obtained  from  the  ordinary  method  of 
slaking,  by  sprinkling,  or  by  immersion,  in  the  case  of  good 
hydraulic  limes,  are  nearly  the  same.  Spontaneous,  or  air- 
slaking,  gives  invariably  the  worst  results.  For  common  and 
slightly  hydraulic  lime,  M.  Yicat  states  that  air-slaking  yields 
the  best  results,  and  ordinary  slaking  the  worst. 

141.  The  ingredients  of  mortar  are  incorporated  either  by 
manual  labor,  or  by  machinery :  the  latter  method  gives  results 
superior  to  the  former.  The  machines  commonly  used  for  mix- 
ing mortar  are  either  the  ordinary  pug-mill  (Fig.  12)  employed 
by  brickmakers  for  tempering  clay,  or  a  grinding-mill  (Fig.  13). 
The  grinding-mill  is  the  best  machine,  because  it  not  only  re- 
duces the  lumps,  which  are  found  in  the  most  carefully  burnt 
stone,  after  the  slaking  is  apparently  complete,  but  it  brings  the 
lime  to  the  state  of  a  unif  c;rm  stiff  paste,  w^hich  it  should  re- 
ceive before  the  sand  is  incorporated  with  it.  The  same 
should  be  done  with  respect  to  the  addition  of  cement,  or  of 
an  alkaline  silicate  to  the  lime  paste,  the  former  in  powder, 
and  the  latter  in  solution,  being  uniformly  sprinkled  over  the 
surface  and  then  thoroughly  incorporated  with  the  other  ma- 
terials by  the  action  of  the  mill.  Care  should  be  taken  not 
to  add  too  much  water,  particularly  when  the  mortar  is  to  be 
immersed  in  water.  The  mortar-mill,  on  this  account,  should 
be  sheltered  from  rain ;  and  the  quantity  of  water  w^th  which 
it  is  supplied  may  vary  with  the  state  of  the  weather.  Noth- 
ing seems  to  be  gained  by  carrying  the  process  of  mixing  be- 
yond obtaining  a  uniform  mass  of  the  consistence  of  plastic 
clay.    Mortai-s  of  hydraulic  lime  are  injured  by  long  expo- 


52 


CIVIL  EXGINEEEING. 


sure  to  the  air,  and  frequent  turnings  and  mixings  with  a 
shovel  or  spade;   those  of  common  lime,  under  like  circum- 


Fig.  12  rejjresents  a  vertical  section  throngh 
the  axiR  of  a  pi;g-mill,  for  mixing  or  tem- 
pering mortar.— This  mill  consists  of  a 
hooped  vessel,  of  the  form  of  a  conical 
frustum,  which  receives  the  ingredients, 
and  a  vertical  shaft,  to  which  arms  with 
teeth,  resembling  an  ordinary  rake,  are 
attached,  for  the  purpose  of  mixing  the 
ingredients. 

A,  A,  section  of  sides  of  the  vessel. 

B,  vertical  shaft  to  which  the  arms  C  are  af- 
fixed. 

D,  horizontal  bar  for  giving  a  circolar  mo- 
tion to  the  shaft  B. 

E,  sills  of  timber  supporting  the  milL 

B,  wrought-iron  support  through  which  the 
upper  part  of  the  shaft  passes. 


stances  seem  to  be  improved.  Mortar  which  has  been  set 
aside  for  a  day  or  two,  will  become  sensibly  firmer;  if  not 


Fig.  13  represents  a  part  of  a  mill  for  crashing  the  lime 

and  tempering  the  mortar. 

A,  heavy  wheel  of  timber,  or  cast  iron. 

B,  horizontal  bar  passing  through  the  wheel,  which  at 
one  extremity  is  fixed  to  a  vertical  shaft,  and  is  ar- 
ranged at  the  other  (C)  with  the  proper  gearing  for 
a  horse. 

D,  a  circular  trough,  with  a  trapezoidal  cross  section 
which  receives  the  ingredients  to  be  mixed.  The 
trough  may  be  from  20  to  30  feet  in  diameter ;  about 
18  inches  wide  at  top,  and  12  inches  deep ;  and  be 
built  of  hard  brick,  stone,  or  timber  laid  on  a  firm 
foimdation. 


allowed  to  stand  too  long,  it  may  be  again  reduced  to  its 
clayey  consistence,  by  simply  pounding  it  with  a  beetle,  with- 
out any  fresh  addition  of  water. 

Fort  Warren  Mortar  Mill— This  mill  (Fig.  14)  which 
was  used  by  Col.  Thayer  in  the  construction  of  Fort  Warren, 
Boston  Harbor,  consists  of  a  circular  trough,  built  of  brick, 
which  was  fifteen  feet  in  diameter,  measured  between  the 
centre  line  of  the  trough,  the  cross  section  of  which  (A)  was 
thirty-three  inches  in  width  at  the  top,  thirteen  inches  at  the 


MOKTAR. 


63 


bottom,  and  twenty-four  inches  deep.  The  brick  side-walls 
(A')  twelve  inches  thick  at  top,  and  built  vertically  on  the  in- 
terior and  outside,  rested  on  an  annular  trench  of  concrete, 


Fig.  14.    Section  through  the  axis  of  the  Fort  Warren  Mortar  Mill. 

A,  Annular  trough  for  mixing  the  mortar. 
A',  Brick  sides  of  trough. 

B,  Central  brick  cylinder. 

C,  Annular  space  for  holding  lime  in  paste. 

D,  Wheel  of  miU. 

E,  Shaft  worked  by  horse  power. 

F,  Wooden  trough  for  conveying  lime  paste  to  0. 

G,  Horse  track. 

one  foot  thick,  which  was  laid  on  an  annular  bed  of  broken 
stone,  two  feet  thick,  for  drainage. 

In  the  centre  of  the  circle  enclosed  by  the  trough,  a  verti- 
cal post,  surrounded  with  broken  stone,  encased  by  a  brick 
cylinder  (B)  has  a  gudgeon  at  top,  around  which  the  horizon- 
tal shaft  (E)  turns,  that  gives  motion  to  the  wheel  (D)  for 
mixing  the  mortar. 

The  wheel  (D)  is  made  of  wood  on  the  sides  and  periphery, 
and  has  an  iron  tire  twelve  inches  broad  and  half  an  inch 
thick ;  the  interior  being  filled  with  sand  to  give  it  sufiicient 
weight  to  grind  any  lumps  in  the  lime  to  a  paste.  The  diam- 
eter of  the  wheel  is  eight  feet,  and  thickness  eight  inches. 

The  radius  of  the  horse  track  for  working  the  wheel  is 
twenty  feet. 

The  annular  space  between  the  trough  and  the  brick  cylin- 
der in  the  centre  is  floored  with  concrete,  resting  on  a  bed  of 
broken  stone. 

Lieut.  W.  H.  Wright,  in  his  Treatise  on  Mortars^  thus  de- 
scribes the  use  made  of  this  annular  ring :  "  The  space  be- 
tween the  cylinder  and  trough  is  used  as  a  reservoir  for  the 
slaked  lime.  It  is  conveniently  divided  by  means  of  movable 
radial  partitions  into  sixteen  equal  parts,"  each  containing  the 
sixteenth  part  of  a  cask  of  lime  in  paste. 

A  wooden  trough  (F)  leads  from  the  reservoir  where  the 


54 


CIVIL  ENGINEERING. 


lime  is  slaked  and  converted  into  a  creamy  consistence,  to  the 
annular  ring  (C),  where  it  is  allowed  to  stand  as  long  as  pos- 
sible before  being  thrown,  with  the  requisite  quantity  of 
sand,  into  the  mill. 

The  malaxator. — Many  advantages  are  claimed  for  a  mill 
designed  by  M.  Coignet,  recently  introduced  in  France,  and 
employed  in  mixing  beton  agglomere  for  the  works  in  and 
about  Paris.  It  is  called  a  malaxator,  and  consists  of  twin 
screws,  Jiaving  their  helices  interlocked,  and  turning  and  ex- 
erting their  force  in  the  same  direction.  This  machine  may 
be  described  as  follows  : 


Fig.  15. 


A  is  the  frame  of  the  machine,  having  at  the  upper  end  the 
cross-pieces  B,  upon  which  are  mounted  the  gearings,  and  at 
the  lower  part  the  cross-piece  c  g\  upon  which  are  fixed  the 
rests  or  steps  for  the  lower  part  of  the  helices  to  run  in. 

D  are  the  cores  of  the  helices,  upon  which  are  fastened 
either  continuous  or  interrupted  blades  S  S  S,  forming  the 
thread  of  the  helix.  Continuous  blades  are  more  generally 
used. 

K  are  wagon -wheels,  mounted  on  an  axle,  which  enable  the 
machine  to  be  transported  thereon,  and  which,  when  the  ma- 
chine is  in  use,  serve  to  maintain  the  malaxator  at  its  proper 


MORTAR. 


55 


inclination  (about  twenty-five  degrees).  The  brace  J  is  used 
to  steady  the  malaxator. 

M  K  m  N^,  gearings  of  any  kind  for  giving  motion  to  the 
helices,  either  by  steam,  horse-power,  or  hand-power ;  ^,  coni- 
cal sleeves  or  stoppers,  adjustable  upon  the  shafts  D,  for  re- 
gulating the  exodus  of  the  artificial  stone  paste,  and  by  re- 
tarding the  same,  increasing  the  pressure  and  malaxation  of 
the  paste  in  the  part  Q'  of  the  machine. 

Q,  body  of  the  malaxator,  corresponding  in  shape  and  size 
to  the  helices. 

P,  receiving  chamber,  where  the  materials  enter  the  mal- 
axator. 

T,  sand  hopper,  with  its  adjustable  register  or  gate  and, 
when  required,  a  sifting  apparatus ;  sliding  gate,  to  allow 
of  the  drainage  of  the  machine. 

S^,  feeding  screws,  working  in  the  lower  part  of  the  two 
hoppers  R',  the  one  for  lime,  the  other  for  sand,  or  any 
other  material  or  substance  to  be  introduced  into  the  artificial 
stone  paste,  and  feeding  the  same  to  the  chamber  P ; 
T  t' t"  r"\  pulleys,  for  chains  or  belts  ^,  for  transmitting  the 
movement  to  the  feeding  screws  S' ;  t'  t'\  spur-wheel 
and  pinion  (changeable  for  others  of  different  relative  speed), 
for  regulating  the  exact  amount  of  the  two  substances  in  the 
hoppers  Y\!  R^,  to  be  delivered,  in  so  many  turns  of  the 
helices,  into  the  receiving  chamber  P. 

Z  is  a  pipe  for  supplying  the  water,  for  which  there  is  an 
overflow  at  W.  The  sand  being  drowned  or  fully  saturated 
in  a  given  proportion,  by  vai-ying  the  overflow  W,  gives  the 
proper  amount  of  water  for  each  turn  of  the  helices. 

H  are  movable  wooden  shafts,  which  are  placed  in  proper 
straps  in  the  machine,  and  serve  to  hitch  or  harness  a  horse  to 
the  same  when  it  has  to  be  taken  from  one  place  to  another, 
making  it  a  perfect  wagon. 

The  advantages  claimed  for  the  malaxator  are  the  following : 

First.  The  apparatus,  having  the  receiving  chamber  P  upon 
the  ground,  is  fed  easily,  with  little  labor  ;  and  the  part  Q', 
or  delivery,  being  elevated,  allows  of  a  wheelbarrow  or  basket 
being  placed  under  to  receive  the  artificial  stone  paste.  This 
inclination  also  causes  a  more  powerful  malaxation,  by  retard- 
ing the  progress  of  the  matter,  owing  to  the  specific  gravity. 

Second.  The  gearings  are  out  of  the  way,  away  from  sand, 
water,  dust,  etc. 

Third.  The  helices  having  their  blades  interlaid,  their 
action  upon  the  materials  is  of  quite  a  different  character  than 
when  said  helices  are  not  thus  conjugated. 


56 


CrVIL  ENGESTEEKING. 


Fourth.  The  sand  is  gauged  by  a  register.  The  lime  and 
the  hydraulic  cement,  the  coloring  matter,  texture  giver,  or 
any  other  material  used,  may  be  also  fed  automically,  and  the 
machine  once  set  by  the  inspector,  the  product  is  invariably 
the  same,  besides  saving  the  labor  of  a  hand  whose  trustwor- 
thiness is  required  to  obtain  good  results.  The  continuous  in- 
troduction by  small  and  regular  quantities  of  the  different 


Fig.  16  represents  a  vertical  section  of  the 
mixincr  cylinder  for  baton  coignet. 

a,  side  of  cj'linder. 

b,  cast  iron  base, 

c,  vertical  shaft. 

d,  a,  curved  arms. 

e,  e,  hclicoidal  blades. 
A  cycloidal  arms. 

Qy  horizontal  opening  at  the  base. 


h,  h,  vertical  gnides  for  movable  band. 

E,  E,  short  stationary  arms. 

G,  (t,  movable  band. 

H,  H,  handles  for  lifting  band. 

I,  supply  trough. 
L,  scraper. 

N,  revolving  horizontal  plate. 
P,  immovable  bottom  plate. 


substances,  and  the  constant  amount  of  the  water  supplied  to 
the  sand,  place  the  materials  in  the  best  circumstances  for 
producing,  by  proper  action  of  the  helices,  an  excellent  result, 


MOKTAE. 


57 


difficult  to  obtain  if  the  component  ingredients  had  been 
thrown  in  by  shovel  or  basketfuls  at  a  time.  (See  Profes- 
sional Papers,  Corps  of  Engineers,  j^o.  19). 

Another  form  of  mill,  which  is  shown  in  Fig.  16,  has  been 
made  use  of  in  France  for  mixing  certain  kinds  of  beton.  It 
consists  of  a  vertical  cylinder  a  resting  on  a  cylindrical  base 
of  cast  iron  h.  A  vertical  shaft  c  passes  through  the  cylinder, 
having  attached  to  it  curved  arms  which,  by  revolving 
horizontally,  serve  to  mix  the  sand  and  lime.  The  distributor 
Q  revolves  horizontally,  receives  the  sand  and  lime  which  come 
from  the  conducting  trough  I,  and  distributes  them  equally 
around  for  mixing.  Short  stationary  arms  E  E  are  attached 
to  the  side  of  the  cylinder,  and  form,  with  the  movable  arms, 
breaks  for  dashing  and  mixing  the  sand  and  lime.  Three 
helicoidal  blades  e  e,  attached  to  the  lower  part  of  the  shaft, 
force  the  mixture  downwards  and  outwards.  Cycloidal  arms 
ff  revolving  horizontally  near  the  floor  of  the  cylinder,  expel 
the  mixture  at  the  side  opening  around  the  bottom.  A  mova- 
ble band  of  iron  G  G,  by  being  moved  up  or  down,  enlarges 
or  diminishes  the  opening  around  the  bottom,  h  A,  vertical 
guiding  shafts  for  movable  band.  H  H,  handles  by  which 
the  band  G  G  is  moved.  A  plate  ]^  is  attached  to  c  and  re- 
volves horizontally,  receiving  the  mixture  from  the  cylinder. 
A  curved  plate  of  iron  L,  fixed  to  immovable  bottom-plate  P, 
scrapes  mixture  from  K  as  it  revolves. 

143.  Setting-  and  durability  of  mortars.  Mortar  of 
common  lime,  without  any  addition  of  puzzolana,  will  not  set  in 
humid  situations,  like  the  foundations  of  edifices,  until  after  a 
very  long  lapse  of  time.  They  set  very  soon  when  exposed 
to  the  air,  or  to  an  atmosphere  of  carbonic  acid  gas.  If,  after 
having  become  hard  in  the  open  air,  they  are  placed  under 
water,  they  in  time  lose  their  cohesion  and  fall  to  pieces. 

144.  Common  mortars,  which  have  had  time  to  harden, 
resist  the  action  of  severe  frosts  very  well,  if  they  are  made 
rather  poor,  or  with  an  excess  of  sand.  The  sand  should  be 
over  2.40  parts,  in  bulk,  to  one  volume  of  the  lime  in  paste ; 
and  coarse  sand  is  found  to  give  better  results  than  fine  sand. 

145.  Good  hydraulic  mortars  set  equally  well  in  damp 
situations,  and  in  the  open  air ;  and  those  which  have  hard- 
ened in  the  air  will  retain  their  hardness  when  immersed  in 
water.  They  also  resist  well  the  action  of  frost,  if  they  have 
had  time  to  set  before  exposure  to  it ;  but,  like  common  mortars, 
they  require  to  be  made  with  an  excess  of  sand,  to  withstand 
well  atmospheric  changes. 

146.  The  surface  of  a  mass  of  hydraulic  mortar,  whether 


68 


CIVIL  ENGINEERING. 


made  of  a  natural  hydraulic  lime  or  otherwise,  when  im- 
mersed in  water,  becomes  more  or  less  degraded  by  the  action 
of  the  water  upon  the  lime,  particularly  in  a  current.  When 
the  water  is  stagnant,  a  very  thin  crust  of  carbonate  of  lime 
forms  on  the  surface  of  the  mass,  owing  to  the  absorption  by 
the  lime  of  the  carbonic  acid  gas  in  the  water.  This  crust, 
if  the  water  be  not  agitated,  will  preserve  the  soft  mortar 
beneath  it  from  the  farther  action  of  the  water,  until  it  has 
had  time  to  become  hard,  when  the  water  will  no  longer  act 
upon  the  lime  in  any  perceptible  degree. 

147.  Ilj^draulic  mortars  set  with  more  or  less  promptness, 
according  to  the  character  of  the  hydraulic  lime,  or  of  the 
puzzolana  which  enters  into  their  composition.  Artificial  hy- 
draulic mortars,  with  an  excess  of  lime,  set  more  slowly  than 
when  the  lime  is  in  a  just  proportion  to  the  other  ingredients. 

143.  The  quick-setting  hydraulic  limes  are  said  to  furnish 
a  mortar  w^iich,  in  time,  acquires  neither  as  much  strength 
nor  hardness  as  that  from  the  slower-setting  hydraulic  limes. 
Artificial  hydraulic  mortars,  on  the  contrary,  which  set  quick- 
ly gain,  in  time,  more  strength  and  hardness  than  those  which 
set  slowl}^ 

149.  The  time  in  which  hydraulic  mortars,  immersed  in 
water,  attain  their  greatest  hardness,  is  not  well  ascertained. 
Mortars  made  of  strong  hydraulic  limes  do  not  show  any 
appreciable  increase  of  hardness  after  the  second  year  of 
their  immersion;  wiiile  the  best  artificial  hydraulic  mortars 
continue  to  harden,  in  a  sensible  degree,  during  the  third  year 
after  their  immersion. 

150.  It  is  found  from  experience  that  those  mortars  which 
attain  the  highest  degree  of  hardness  on  the  surface,  absorb 
the  least  amount  of  water  and  are  less  liable  to  injury  from 
frost  and  weather. 

151.  Theory  of  Mortars.  The  paste  of  a  hydrate,  either 
of  common  or  of  hydraulic  lime,  when  exposed  to  the  air,  ab- 
sorbs carbonic  acid  gas  from  it ;  passes  to  the  state  of  sub- 
carbonate  of  lime;  without,  however,  rejecting  the  water  of 
the  hydrate,  and  gradually  hardens.  The  time  required  for 
the  complete  saturation  of  the  mass  exposed,  will  depend  on 
its  bulk.  The  absorption  of  the  gas  commences  at  the  surface 
and  proceeds  more  slowly  towards  the  centre.  The  harden- 
ing of  mortars  exposed  to  the  atmosphere  is  generally  attrib- 
uted to  this  absorption  of  the  gas,  as  no  chemical  action  of 
lime  upon  quartzose  sand,  which  is  the  usual  kind  employed 
for  mortars,  has  hitherto  beep  detected  by  the  most  careful 
experiments. 


CONCRETE. 


59 


The  depth  to  which  the  absorption  of  carbonic  acid  ex- 
tends in  hydraulic  lime,  and  also  in  some  degree  the  hardening, 
decreases  as  the  hydraulic  energy  caused  by  the  silica  that 
enters  into  their  composition  is  the  greater. 

152.  With  regard  to  hydraulic  mortars,  it  is  difficult  to  ac- 
count for  their  hardening,  except  upon  the  effect  which  the 
silicate  of  lime  may  have  upon  the  excess  of  simple  hydrate 
of  uncombined  lime  contained  in  the  mass.  M.  Petot  sup- 
poses, that  the  particles  of  silicate  of  lime  form  so  many 
centres,  around  which  the  uncombined  hydrates  group  them- 
selves in  a  crystalline  form ;  becoming  thus  sufficiently  hard 
to  resist  the  solvent  ac-tion  of  water.  With  respect  to  the 
action  of  quartzose  sand  in  hydraulic  mortars,  M.  Petot 
thinks  that  the  grains  produce  the  same  mechanical  effect  as 
the  particles  of  the  silicate  of  lime,  in  inducing  the  aggrega- 
tion of  the  uncombined  hydrate. 


Y. 

CONCRETE.  BETON. 

153.  This  term  is  applied,  by  English  architects  and  engi- 
neers, to  a  mortar  of  finely-pulverized  quick-lime,  sand,  and 
gravel.  These  materials  are  first  thoroughly  mixed  in  a  dry 
state,  sufficient  water  is  added  to  bring  the  mass  to  the  ordi- 
nary consistence  of  mortar,  and  it  is  then  rapidly  worked  up 
by  a  shovel,  or  else  passed  through  a  pug-mill.  The  concrete 
is  used  immediately  after  the  materials  are  well  incorporated, 
and  while  the  mass  is  hot. 

154.  The  materials  for  concrete  are  compounded  in  various 
proportions.  The  most  approved  are  those  in  which  the  lime 
and  sand  are  in  the  proper  proportions  to  form  a  good  mortar, 
and  the  gravel  is  twice  the  bulk  of  the  sand.  The  gravel 
used  should  be  clean,  and  any  pebbles  contained  in  it  larger 
than  an  egg,  should  be  broken  up  before  the  materials  are 
incorporated. 

155.  Hot  water  has  in  some  cases  been  used  in  making 
concrete.  It  causes  the  mass  to  set  more  rapidly,  but  is  not 
otherwise  of  any  advantage. 


60 


CIYIL  ENGINEERrNG. 


156.  The  bulk  of  a  mass  of  concrete,  when  first  made,  is 
found  to  be  about  one-fifth  less  than  the  total  bulk  of  the  dry 
materials.  But,  as  the  lime  slakes,  the  mass  of  concrete  is 
found  to  expand  about  three-eighths  of  an  inch  in  height,  for 
every  foot  of  the  mass  in  depth. 

157.  The  use  of  concrete  is  at  present  mostly  restricted  to 
forming  a  solid  bed,  in  bad  soils,  for  the  foundations  of  edi- 
fices. It  has  also  been  used  to  form  blocks  of  artificial  stone, 
for  the  walls  of  buildings  and  other  like  purposes ;  but  ex- 
perience has  shown  that  it  possesses  neither  the  durability 
nor  strength  requisite  for  structures  of  a  permanent  character, 
when  exposed  to  the  action  of  water,  or  of  the  weather. 

158.  BETON.  The  term  beton  is  applied,  by  French 
engineers,  to  any  mixture  of  hydraulic  mortar  with  fragments 
of  brick,  stone,  or  gravel ;  and  it  is  now  also  used  by  English 
engineers  in  the  same  sense. 

159.  The  proportions  of  the  ingredients  used  for  beton  are 
variously  stated  by  different  authors.  The  sole  object  for 
which  the  gravel,  or  the  broken  stone  is  used,'  being  to  obtain 
a  more  economical  material  than  a  like  mass  of  hydraulic 
mortar  alone  would  yield,  the  quantity  of  broken  stone  should 
be  as  great  as  can  be  thoroughly  united  by  the  mortar.  The 
smallest  amount  of  mortar,  therefore,  that  can  be  used  for 
this  purpose,  will  be  that  which  will  be  just  equal  in  volume 
to  the  void  spaces  in  any  given  bulk  of  the  broken  stone,  or 
gravel.  The  proportion  which  the  volume  occupied  by  the 
void  spaces  bears  to  any  bulk  of  a  loose  material,  like  broken 
stone,  or  gravel,  may  be  readily  ascertained  by  filling  a  vessel 
of  known  capacity  with  the  loose  material,  and  pouring  in  as 
much  water  as  the  vessel  will  contain.  The  volume  of  water 
thus  found,  will  be  the  same  as  that  of  the  void  spaces. 

Beton  made  of  mortar  and  broken  stone,  in  which  the  pro- 
portions of  the  ingredients  were  ascertained  by  the  process 
just  detailed,  has  been  found  to  give  satisfactory  results ;  but, 
in  order  to  obviate  any  defect  arising  from  imperfect  manip- 
ulation, it  is  usual  to  add  an  excess  of  mortar  above  that  of 
the  void  spaces. 

160.  In  a  large  amount  of  concrete  used  for  the  foundation 
bed  and  backing  of  the  sea  walls  built  for  the  protection  of 
the  islands  in  Boston  Harbor,  which  was  composed  of  hydrau- 
lic mortar  made  with  salt  water  and  the  common  shingle  of 
the  shores,  which  varied  in  size  from  that  of  a  pea  to  pebbles 
of  six  inches  in  diameter,  the  proportions  used  for  the  foun- 
dation bed  was  about  one  part  in  volume  of  stiff  mortar  to  three 


CONCRETE. 


61 


parts  in  volume  of'  shingle  for  the  foundation  bed,  and  two 
and  seven-tenths  parts  for  the  backing  of  the  walls.  The 
small  and  large  pebbles  of  the  shingle  were  so  proportioned 
as  to  give  the  least  amount  of  void  space  to  be  filled  by  the 
mortar ;  this  void  space  being  from  twenty  to  twenty-five  per 
cent,  of  the  volume  of  shingle. 

The  materials  were  mixed  by  hand ;  the  shingle  first  being 
spread  out  upon  a  platform  of  rough  boards  to  the  depth  of 
from  eight  to  twelve  inches,  the  larger  pebbles  on  top ;  the 
mortar  was  spread  in  a  layer  of  uniform  thickness  over  this, 
and  the  whole  worked  up  with  shovels  and  hoes  until 
thoroughly  incorporated. — (Papers  on  Practical  Engineering, 
No.  2.    Report  of  Col.  S.  Thayer,  U.  S.  Corps  of  Engineers.) 

In  the  hydraulic  concrete  used  upon  some  others  of  our 
public  works,  the  broken  fragments  of  granite  were  in  bulk 
about  If  that  of  the  hydraulic  mortar.  Besides  this,  other 
fragments,  from  a  quarter  to  three-quarters  of  a  cubic  foot  each, 
and  forming  about  one-twelfth  of  the  volume  of  the  concrete, 
were  worked  into  the  layer  as  they  were  carried  up.  This 
practice  is  a  very  usual  one  for  foundation  beds,  as  it  effects  a 
saving  of  cost. 

The  best  and  most  economical  beton  is  made  of  a  mixture 
of  broken  stone,  or  brick,  in  fragments  not  larger  than  a 
hen's  egg,  and  of  coarse  and  fine  gravel  mixed  in  suitable 
proportions. 

In  making  beton,  the  mortar  is  first  prepared,  and  then  in- 
corporated with  the  finer  gravel ;  the  resulting  mixture  is 
spread  out  into  a  cake,  4  or  6  inches  in  thickness,  over  which 
the  coarser  gravel  and  broken  stone  are  uniformly  strewed 
and  pressed  down,  the  whole  mass  being  finally  brought  to  a 
homogeneous  state  with  the  hoe  and  shovel. 

Beton  is  used  for  the  same  purposes  as  concrete,  to  which 
it  is  superior  in  every  respect,  but  particularly  so  for  foun- 
dations laid  under  water,  or  in  humid  localities. 

161.  Beton  made  of  small  fragments  of  stone  or  pebbles 
has  within  recent  years  been  applied  to  the  construction  of  the 
walls  of  houses.  For  this  purpose,  the  concrete  is  laid  up  in 
layers  and  rammed  within  a  plank  boxing  having  an  interior 
width  equal  to  the  thickness  of  wall.  The  sides  of  the  boxing 
are  confined  by  vertical  posts  which  can  be  suitably  adjusted 
to  the  required  thickness  of  the  wall ;  the  whole  being  sup- 
ported by  a  suitable  scaffolding.  In  the  case  of  hollow  walls, 
a  slip  of  board  of  the  thickness  of  the  required  hollow,  or 
void,  and  slightly  wedge-shaped  to  admit  of  its  being  easily 
removed,  is  laid  horizontally  within  the  box,  and  the  layer  of 


6^ 


CIVIL  ENGINEERING. 


concrete  rammed  well  in  around  it ;  ordinary  brick  being  in- 
serted as  ties  to  connect  the  interior  and  exterior  portions  of 
the  wall. 

In  the  sewers  and  many  public  and  private  edifices  recently 
constructed  in  Paris  of  concrete,  the  proportions  used  were 
one  part  in  volume  of  lime,  one  fourth  of  one  volume  of 
hydraulic  cement,  to  five  volumes  of  sand.  It  is  stated  that  in 
six  or  eight  hours  after  beginning  a  given  length  of  sewer  the 
centres  can  be  safely  removed ;  and  that,  in  four  or  five  days 
after  a  section  has  been  completed,  it  can  be  opened  for  use. 
For  the  construction  of  arches,  the  volume  of  cement  used  is 
doubled. 

Some  of  the  buildings  above  referred  to  were  constructed 
with  groined  or  cylindrical  arched  fire-proof  floors,  of  spans 
from  nine  to  twenty-eight  feet,  the  rise  in  each  case  being  one 
tenth  of  the  span ;  the  thickness  of  the  arches,  at  the  crown, 
varying  from  five  and  a  half  to  fourteen  inches. 

The  crushing  weight  of  this  concrete  is  nearly  fifty-four 
hundred  pounds  to  the  square  inch ;  the  tenacity  about  five 
hundred  pounds. 

162.  An  artificial  sandstone,  termed  Beton-Coignet  from 
the  inventor,  is  very  extensively  manufactured  and  used  in 
France  for  all  building  purposes,  as  foundations,  walls,  light 
arches,  etc.  It  sets  and  hardens  in  a  comparatively  short  time. 
Its  constituents  are  clean  river  sand  from  four  to  five  parts  in 
volume ;  common  or  hydraulic  lime  one  part  in  volume ; 
hydraulic  or  artificial  Portland  cement  from  one-quarter  to 
three-quarters  of  one  part  in  volume  ;  water  variable,  but  only 
enough  to  moisten  the  other  materials  and  cau«e  them  to 
cohere.  Coarse  sand  from  one-twentieth  to  three-twentieths 
of  an  inch  in  diameter  is  said  to  give  the  best  results ;  the 
finer  sands  requiring  more  care  in  the  preparation  of  the 
concrete  and  in  packing  it  when  laid  to  secure  greater  so- 
lidity. 

163.  In  preparing  the  concrete  the  lime  and  sand  are  made 
into  heaps  of  about  one  cubic  yard  in  volume  in  alternate 
layers  of  the  two  ingredients.  Each  heap  is  then  worked  up 
dry  with  the  shovel.  In  this  state  it  is  delivered  by  suitable 
machinery,  like  that  for  raising  grain,  into  the  top  of  a  pug- 
mill  of  a  cylindrical  body  formed  of  boiler  iron.  The  revolv- 
ing vertical  shaft  of  the  mill,  which  is  driven  by  steam  or 
animal  power,  has  curved  arms  affixed  horizontally  to  it,  the 
two  lower  arms  being  of  suitable  forms  to  press  the  mixed 
material  downwards,  and  expel  it  through  an  a])erture,  where 
it  is  received  into  boxes,  or  hand  barrows,  and  conveyed  to 


CONCEETE. 


63 


where  it  is  to  be  laid  or  moulded.  The  water  for  the  mixing 
is  either  thrown  in  as  needed,  by  hand  into  the  top  of  the 
mill,  or  else  supplied  by  a  circular  trough  perforated  with 
holes,  which  is  placed  around  the  inside  of  the  mill  at  top. 
When  cement  is  one  of  the  ingredients,  it  is  fir^  made  into  a 
suitable  paste  with  water,  and  then  added  to  the  others,  from 
a  vessel  over  the  top  of  the  mill,  from  which  it  is  poured  in  a 
uniform  manner,  and  in  the  requisite  amount. 

164.  For  all  ordinary  workj  one  passage  through  the  pug- 
mill  is  sufficient,  but  where  greater  thoroughness  in  the  mix- 
ture is  a  requisite,  the  concrete  may  be  passed  through  the 
mill  a  second  time. 

165.  The  concrete  when  laid  or  moulded  is  put  in  in  suc- 
cessive layers,  from  one  to  three  inches  in  thickness,  and 
packed  moderately  by  hand .  with  pestles  weighing  from  fif- 
teen to  thirty  pounds. 

166.  To  increase  the  rapidity  of  the  setting,  when  necessary, 
the  materials  may  be  heated,  in  process  of  mixing,  by  a  spi- 
ral tube  or  worm,  through  which  heated  air,  steam,  or  hot 
water  is  caused  to  circulate. 

167.  Among  other  artificial  conglomerates,  that  known  as 
Eansome's  artificial  stone,  from  the  name  of  the  inventor,  is 
now  coming  into  use  in  England.  This  material  consists  of 
clean  river  sand  the  grains  of  which  are  cemented  with  the 
silicate  of  lime.  To  effect  this  union  a  silicate  of  soda  is 
formed,  by  digesting  common  flints  in  a  solution  of  caustic 
soda,  in  iron  air-tight  cylindrical  vessels,  by  means  of  steam, 
under  a  pressure  of  seventy  pounds,  which  circulates  through 
a  coil  of  iron  pipes.  The  sand,  after  being  thoroughly  dried, 
is  mixed  with  a  sufficient  volume  of  finely  ground  carbonate 
of  lime  to  fill  the  voids  between  the  grains.  To  each  bushel 
of  this  mixture  a  gallon  of  the  silicate  is  added,  and  the 
whole  thoroughly  mixed  in  a  loam  mill.  The  mixture  is  then 
moulded,  and  immediately  after  the  solution  of  the  chloride 
of  calcium  is  thrown  over  it  with  ladles ;  the  moulded  blocks 
are  then  immersed  in  the  solution,  in  open  tanks,  which  is 
•kept  boiling,  by  steam  passed  through  it  in  pipes,  for  several 
hours,  according  to  the  size  of'  the  blocks.  This  process  ex- 
pels any  air  that  may  have  been  retained  in  the  blocks  and 
facilitates  the  forming  of  the  silicate  of  calcium.  "  Tlie  block 
is  then  taken  out  and  the  chloride  of  sodium,  that  has  been 
formed,  thoroughly  washed  out  with  fresh  water  poured  over 
the  block. 

This  artificial  stone  is  found  to  be  very  liard,  and  some 
specimens  to  have  offered  as  great  a  resistance  to  rupture,  by 


6^ 


CIVIL  ENGINEEEINa. 


compression  and  extension,  as  the  best  sandstones  and  mar- 
bles. 

168.  General  Gillmore  in  his  Report,  Professional  Papers, 
Corjps  of  Engineers^  No.  19,  gives  the  following  account  of 
beton-Coign<^  or  agglomere. 

Beton  Agglomere.  This  name  is  given  to  a  beton  of 
very  superior  quality,  or,  more  properly  speaking,  an  artificial 
stone  of  great  strength  and  hardness,  which  has  resulted  from 
the  experiments  and  researches,  extending  through  many 
years,  of  M.  rran9ois  Coignet,  of  Paris. 

The  essential  conditions  which  must  be  carefully  observed 
in  making  this  beton  are  as  follows : 

First.  Only  materials  of  the  first  excellence  of  their  kind, 
"whether  common  or  hydraulic  lime,  or  hydraulic  cement,  can 
be  used  for  the  matrix. 

Second.  The  quantity  of  water  must  not  exceed  what  is 
barely  sufficient  to  convert  the  matrix  into  a  stiff,  \ascous  paste. 

Third.  The  matrix  must  be  incorporated  with  the  solid 
ingredients  by  a  thorough  and  prolonged  mixing  or  trituration, 
producing  an  artificial  stone  paste,  decidedly  incoherent  in 
character  until  compacted  by  pressure,  in  which  every  grain 
of  sand  and  gravel  is  completely  coated  with  a  thin  film  of 
the  paste.  There  must  be  no  excess  of  paste  when  the  matrix 
is  common  lime  alone.  With  hydraulic  lime  this  precaution 
is  less  important,  and  with  good  cement  it  is  unnecessary. 

Fourth.  The  beton  or  artificial  stone  is  formed  by  thorough- 
ly ramming  the  stone  paste,  in  thin,  successive  layers,  with 
iron-shod  rammers. 

169.  The  materials  employed  in  making  his  beton  are 
sand,  common  lime,  hydraulic  lime,  and  Portland  cement. 

The  sand  should  be  as  clean  as  that  ordinarily  required  for 
mortar,  for  stone  or  brick  masonry  of  good  quality.  Sand 
containing  5  or  6  per  cent,  of  clay  may  be  used  without 
washing,  for  common  work,  by  proportionally  increasing  the 
amount  of  matrix.  Either  fine  or  coarse  sand  will  answer, 
or,  preferably,  a  mixture  of  both,  containing  gravel  as  large 
as  a  small  pea,  and  even  a  small  proportion  of  pebbles  as- 
large  as  a  hazel  nut.  There  is  an  advantage  in  mixing 
several  sizes  together,  in  such  proportion  as  shall  reduce  the 
volume  of  voids  to  a  minimum.  Coarse  sand  makes  a  harder 
and  stronger  beton  than  fine  sand.  The  extremes  to  be 
avoided  are  a  too  minute  subdivision  and  weakening  of  the 
matrix,  by  the  use  of  fine  sand  only,  on  the  one  hand,  and  an 
undue  enlargement  of  tlie  volume  of  voids,  by  the  exclusive 
use  of  coarse  sand,  on  the  other. 


CONOKETE. 


65 


The  silicious  sands  are  considered  the  best,  though  all 
kinds  are  employed.  When  special  results  are  desired  in  the 
way  of  strength,  texture,  or  color,  the  sand  should  be  selected 
accordingly. 

170.  The  common  lime  should  be  air-slaked,  or,  better 
still,  it  may  be  slaked  by  aspersion  with  the  minimum 
quantity  of  water  that  will  reduce  it  to  an  impalpable  pow- 
der, it  should  be  passed  through  a  line  wire  screen  to 
exclude  all  lumps,  and  used  within  a  day  or  two  after 
slaking,  or  else  kept  in  boxes  or  barrels  protected  from  the 
atmosphere. 

It  is  scarcely  practicable,  under  ordinary  circumstances, 
to  employ  fat  lime  alone  as  the  matrix  of  beton  agglomere, 
particularly  in  monolithic  constructions,  in  consequence  of  its 
tardy  induration.  Even  when  used  in  combination  with 
hydraulic  lime  or  cement  it  acts  as  a  diluent. 

171.  Attempts  to  make  beton  of  even  average  quality, 
without  good  hydraulic  ingredients,  have  failed  in  the  LTnited 
States ;  and  it  is  extremely  doubtful  whether  any  character- 
istic excellence  can  be  attained,  after  the  lapse  of  weeks  or 
even  months,  by  a  mixture  of  this  character. 

172.  The  most  suitable  hydraulic  limes  are  those  derived 
from  the  argillaceous  limestones,  in  contradistinction  to  the 
magnesian  or  argillo-magnesian  varieties.  These  limestones 
contain  before  burning  from  15  to  25  per  cent. — generally 
less  than  20  per  cent.  — of  clay.  After  burning,  the  lime  is 
slaked  to  powder  by  aspersion  with  water,  and  sifted  to 
exclude  unslaked  lumps. 

Hydraulic  lime  cannot  be  considered  an  essential  ingre- 
dient of  beton  agglomere,  except  in  comparison  with  common 
lime.  It  may  be  altogether  replaced  by  good  hydraulic 
cement,  or  it  may  be  used  alone,  or  mixed  with  common  lime, 
to  the  entire  exclusion  of  cement.  A  stiff  paste  of  this  lime 
should  set  in  the  air  in  from  ten  to  fifteen  hours,  and 
sustain  a  wire  point  one-twenty-fourth  of  an  inch  in  diameter, 
loaded  with  one  pound,  in  eighteen  to  twenty-four  hours. 
Its  energ}^,  and  therefore  its  value,  varies  directly  with  the 
amount  of  clay  which  it  contains,  wliicli  generally  will  not 
exceed  20  per  cent,  before  burning,  although  it  may  reach  25 
per  cent.  Beyond  this  point  the  burnt  stone  can  seldom  be 
reduced  by  slaking  and  becomes  a  cement. 

ISTo  hydraulic  lime  of  this  variety  has  ever  been  manufac- 
tured in  the  United  States.  It  is  not  known  that  stone  suit- 
able for  it  exists  here. 

173.  The  heavy  slow-setting  Portland  cements,  natural  or 

6 


66 


CIYIL  ENGINEEEINQ. 


artificial,  are  the  only  ones  suitable  for  beton  agglomere. 
They  are  manufactured  extensively  throughout  Europe. 

This  cement  is  produced  by  burning,  with  a  heat  of  great 
intensity  and  duration,  argillaceous  limestones,  containing 
from  20  to  22  per  cent,  of  clay,  or  an  artificial  mixture  of 
carbonate  of  lime  and  clay  in  similar  proportions,  and  then 
reducing  the  product  to  fine  powder  between  millstones.  In 
this  condition  its  weight  should  not  fall  short  of  101  pounds 
and  will  seldom  exceed  128  pounds  to  the  bushel,  poured  in 
loosely  and  struck,  without  being  shaken  down  or  compacted. 
Between  these  limits  additional  weight  may  always  be  con- 
ferred in  the  burning,  by  augmenting  the  intensity  and 
duration  of  the  heat ;  and  both  the  tensile  strength,  and  the 
time  required  to  set^  increase  directly  with  the  weight.  For 
example,  a  Portland  cement  weighing  100  pounds  to  the 
United  States  bushel,  that  will  set  in  half  an  hour,  and  sus- 
tain when  seven  days  old  a  tensile  strain  of  200  pounds  on  a 
sectional  area  of  one  square  inch,  would  have  its  time  for 
setting  increased  to  four  or  five  hours,  and  its  tensile  strength 
to  about  400  pounds,  if  burnt  to  weigh  124  pounds  to  the 
bushel.  An  increase  in  weight  of  24  pounds  to  the  bushel 
nearly  doubles  the  ultimate  tensile  strength  of  Portland 
cement. 

When  the  matrix  of  beton  agglomere  is  Portland  cement 
alone,  it  is  customary  to  prolong  the  process  of  trituration,  in 
order  to  retard  the  set ;  or,  if  more  convenient,  the  mixture 
may  be  passed  through  the  mill  twice  or  even  three  times, 
with  an  interval  of  an  hour  or  more  between  each  mixing. 
This  course  is  specially  desirable  when  the  cement  weighs 
less  than  100  hundred  pounds  to  the  bushel,  and  i§  correspond- 
ingly quick-setting. 

174.  English  engineers  generally  require  that  the  cement 
shall  be  ground  so  fine  that  at  least  90  per  cent,  of  it  shall 
pass  a  No.  30  wire  sieve,  of  3G  wires  to  the  lineal  inch,  and 
shall  weigh  not  less  than  106  pounds  to  the  struck  bushel, 
when  loosely  poured  into  the  measure.  When  made  into  a 
stiff  paste  without  sand,  it  should  be  capable  of  sustaining 
without  rupture  a  tensile  strain  of  400  pounds  on  a  sectional 
area  \\  inch  square,  or  2^  square  inches  (equal  to  178 
pounds  to  the  sectional  square  inch),  seven  days  after  being 
moulded  the  sample  being  immersed  six  of  these  days  in 
fresh  water. 

175.  Experience  has  repeatedly  demonstrated,  and  they 
have  become  well  recognized  facts,  that  in  order  to  obtain 
uniformly  good  beton  or  artificial  stone,  with  sand,  and 


CONCRETE. 


67 


either  hydraulic  lime  or  Portland  cement,  or  both,  it  is  neces- 
sary— 

First.  To  regulate,  in  a  systematic  manner,  the  amount  of 
water  employed  in  the  manufacture  thereof. 

Second.  To  obtain,  with  a  minimum  quantity  of  water,  the 
cementing  material  or  matrix  in  a  state  of  plastic  or  viscous 
paste. 

Third.  To  cause  each  grain  of  sand  or  gravel  to  be  entire- 
ly lubricated  with  a  thin  film  or  coating  of  this  paste  ;  and 

Fourth.  To  bring  each  and  every  grain  into  close  and  inti- 
mate contact  with  those  which  surround  it. 

It  is  also  equally  true,  that  the  best  results  possible  to  be 
produced  from  any  given  materials  will  be  attained  when  the 
above-named  conditions  are  enforced. 

176.  It  is  impossible  to  produce  a  cementing  material,  of 
suitable  quality  for  beton  agglomere,  by  the  ordinary  meth- 
ods and  machinery  used  for  making  mortars ;  for  if  we  take 
the  powder  of  hydraulic  lime  or  Portland  cement,  and  add 
the  quantity  of  water  necessary  to  convert  it  into  a  paste  by 
the  usual  treatment,  it  will  usually  contain  so  much  moisture, 
even  after  being  incorporated  with  the  sand,  that  it  cannot  be 
compacted  by  ramming,  but  will  yield  under  the  repeated 
blows  of  the  rammer  like  jelly.  If  the  quantity  of  water  be 
reduced  to  that  point  which  would  render  the  mixture,  with 
the  usual  treatment,  susceptible  of  being  thoroughly  compact- 
ed by  rammers,  nmch  of  the  cementing  substance  will  re- 
main more  or  less  inert,  and  will  perform  but  indifferently 
well  the  functions  of  a  matrix. 

177.  To  prepare  the  matrix,  there  is  taken  of  the  hydrau- 
lic lime  or  cement  powder,  say  one  hundred  parts,  by  meas- 
ure, and  of  water  from  thirty  to  thirty-five  or  forty  parts, 
which  should  be  the  smallest  amount  that  will  accomplish 
the  object  in  view.  These  are  introduced  together  into  a 
suitable  mill,  acting  upon  the  materials  by  both  compression 
and  friction,  and  are  subjected  to  a  thorough  and  prolonged 
trituration,  until  the  result  is  a  plastic,  viscous,  and  sticky 
paste,  of  a  peculiar  character,  in  both  its  physical  appearance 
and  the  manner  in  which  it  comports  itself  under  the  subse- 
quent treatment  with  rammers.  There  would  appear  to  be 
no  mystery  in  this  part  of  the  process,  yet  the  excellence  of 
the  beton  agglomere  is  greatly  dependent  on  its  proper 
execution. 

If  too  much  water  be  used,  the  mixture  cannot  be  suitably 
*  rammed  ;  if  too  little,  it  will  be  deficient  in  strength. 

178.  The  sand  should  be  deprived  of  surplus  moisture, 


68 


CIVIL  ENGINEERING. 


altliough  it  is  not  necessary  that  it  be  absolutely  dry.  A  uni- 
form state  of  moisture  or  flryness  should  be  aimed  at,  in 
order  that  the  proper  quantity  of  water  may  be  added  with 
certainty. 

179.  The  matrix  in  paste,  and  the  sand,  having  been  mix- 
ed together  in  the  desired  proportions  (given  hereafter),  are 
then  introduced  into  a  powerful  mill,  and  subjected  to  a 
thorough  and  energetic  trituration  until,  without  the  addition 
of  more  water,  the  paste  presents  the  desired  degree  of  homo- 
geneity and  plasticity. 

When,  for  any  special  purpose,  it  is  desired  to  introduce 
into  the  mixture  a  quantity  of  Portland  cement,  in  order  to 
increase  the  hardness  or  the  rapidity  of  induration,  it  had 
better  be  added  during  the  process  of  trituration,  mixed  with  the 
requisite  increment  of  water,  so  that  after  proper  mixing  the 
whole  material  Avill  present  the  appearance  of  a  short  paste, 
or  pasty  powder,  which  is  quite  characteristic  of  this  process 
of  manipulation. 

In  ordinary  practice,  when  sand  and  hydraulic  lime  only 
are  employed,  it  will  be  found  to  answer  very  well  to  mix 
the  two  together  dry,  with  shovels,  and  then  spread  them  out 
on  the  floor  .and  sprinkle  them  with  the  requisite  minimum 
amount  of  water.  The  dampened  mixture  is  then  shoveled 
into  the  mill  and  triturated,  as  already  described. 

When  a  portion  of  Portland  cement  is  used,  it  may  also  be 
incorporated  with  the  other  ingredients  before  the  water  is 
added,  or  introduced  into  the  mixture  in  the  mill,  as  may  be 
preferred. 

When  Portland  alone  is  used  for  the  matrix,  the  process  is 
the  same  as  when  lime  alone  is  used,  except  that  the  tritura- 
tion should  be  more  prolonged,  especially  if  the  cement  be 
rather  light  and  quick-setting. 

Having  both  equally  at  command,  the  following  propor- 
tions are  employed  for  divers  purposes,  according  to  circum- 
stances and  the  quality  of  the  materials : 


6 

5 

4 

5 

5 

4 

4 

5 

5 

5 

Hydraulic  lime  in    powder,  by 

1 

1 

1 

1 

1 

1 

1 

1 

1 

1 

Portland  cement  in  powder,  by 

0 

0 

0 

i 

i 

i 

i 

1 

n 

It  will  rarely  occur  that  the  proportions  given  in  the  two 
columns  on  the  right  of  the  above  table  need  be  used.  They 


CONCRETE. 


69 


are  suitable  for  ornamented  blocks,  requiring  removal  and 
handling  a  day  or  two  after  being  made. 

It  may  sometimes  happen  that  too  much  water  has  been 
introduced  in  the  preparation  of  the  paste.  A  proper  correct- 
ive, in  sucli  case,  is  the  introduction  into  the  mill  of  a  suitable 
quantity  of  each  of  the  ingredients,  mixed  together  dry  in  the 
required  proportions. 

By  employing  none  but  white  sand  and  the  lighter-colored 
varieties  of  lime  and  cement,  a  stone  closely  imitating  w^hite 
marble  may  be  made,  while,  by  the  introduction  of  coloring 
matter  into  the  paste,  such  as  ochres,  oxides,  carbonates,  etc., 
or  fragments  of  natural  stones,  any  variations  in  sliade  or  tex- 
ture may  be  produced,  from  the  most  delicate  buff  and  drab, 
to  the  darkest  grays  and  browns. 

In  some  cases  it  may  be  foimd  more  convenient  to  measure 
the  ingredients  directly  into  the  mill,  alternating  with  the 
different  materials,  in  regular  order,  using  for  the  purpose 
measures  of  various  sizes,  corresponding  with  the  required 
proportions. 

When  it  is  specially  desirable  to  obtain  stone  of  the  maxi- 
mum degree  of  strength  and  hardness,  the  paste  may  be  re- 
turned a  second  or  even  a  third  time  to  the  mill,  but  in  all 
cases  the  mass  must  be  brought  to  the  characteristic  state  of 
incoherent  pasty  powder,  or  short  paste. 

180.  The  materials,  after  being  mixed  to  a  state  of  pasty 
powder,  have  to  be  agglomerated  in  moulds,  in  order  to  become 
beton  or  artificial  stone.  In  other  words,  the  grains  of  sand 
and  gravel,  each  coated  all  over  with  a  thin  film  of  the  matrix 
— entirely  exhausting  the  matrix  thereby — have  to  be  brought 
into  close  and  intimate  contact  with  each  other.  This  is  ac- 
complished by  ramming  the  paste  in  thin,  successive  layers, 
in  a  moidd  of  the  form  and  dimensions  required  for  the  stone, 
and  made  so  as  to  be  capable  of  sustaining  heavy  pressure 
from  within,  and  of  being  taken  apart  at  pleasure. 

Into  this  mould,  supposing  it  to  be  for  a  detached  building 
block,  and  not  for  monolithic  masonry,  a  quantity  of  the  stone 
paste  is  thrown  with  a  shovel,  and  spread  out  in  a  layer  from 
li  to  2  inches  thick.  It  is  then  thoroughly  compacted  by  the 
repeated  and  systematic  blows  of  an  iron-shod  rammer,  until 
the  stratum  of  material  is  reduced  to  about  one-third  its  origi- 
nal thickness.  When  this  is  done,  its  surface  is  scratched  or 
roughened  up  with  an  iron  rake,  in  order  to  secure  a  perfect 
bond  with  the  succeeding  stratum,  and  more  of  the  material  is 
added  and  packed  in  the  same  manner.  This  process  is  con- 
tinued until  the  mould  is  full.    The  upper  surface  is  then 


70 


CIVIL  ENGINEEEmG. 


struck  with  a  straight-edge,  and  smoothed  off  with  a  trowel, 
after  which  the  full  mould  may  at  once  be  turned  over  on  a 
bed  of  sand,  and  the  bottom,  side,  and  end  pieces  removed. 
The  block  is  then  finished.  If  small,  such  as  one  man 
can  handle,  it  may  be  safely  removed  after  one  day.  Larger 
pieces,  like  sills,  lintels,  steps,  platforms,  etc.,  should  be  allowed 
a  longer  time  to  harden,  in  consequence  of  their  greater 
weight. 

In  case  of  monolithic  masonry,  the  moulds  usually  consist 
of  a  series  of  planks  placed  one  above  the  other  horizontally, 
and  supported  against  exterior  uprights,  so  arranged  as  to  give 
the  required  form  to  the  work  under  construction.  These 
planks  are  raised  up  as  the  wall  progresses,  so  that  each  day's 
work  shall  unite  intimately  witli  that  of  the  previous  day,  pro- 
ducing a  smooth  and  even  surface,  without  joints,  ridges,  or 
marks  of  any  kind. 

A  characteristic  property  of  this  stone  paste,  when  prop- 
erly mixed,  is  that  it  does  not  assume  a  jelly-like  motion  when 
rammed. 

Its  degree  of  moisture  must  be  precisely  such  that  the  effect 
of  each  blow  of  the  rammer  shall  be  distinct,  local,  and  per- 
manent, without  disturbing  the  contiguous  material  compacted 
by  pi-evious  blows.  If  it  be  too  moist,  the  mass  will  shake 
like  wet  clay,  and  if  it  be  too  dry,  it  will  break  up  around  the 
rammer  like  sand.  In  either  case  the  materials  cannot  be 
compacted  and  agglomerated  in  that  manner  and  to  that 
degree  which  is  characteristic  of,  and  peculiar  to,  beton  agglo- 
mere. 

In  monolithic  buildings  of  this  beton,  it  is  customary  to 
construct  all  the  flues,  pipes,  and  other  openings  for  heating 
and  ventilating,  and  for  conveying  water,  gas,  and  smoke,  in 
the  thickness  of  the  wall,  by  using  movable  cores  of  tlie  re- 
quired size  and  form,  around  which  the  material  is  packed. 
As  the  work  progresses  the  cores  are  moved  up. 

Ornamental  work  of  simple  design  may  be  placed  upon  the 
exterior  of  the  building,  by  attaching  the  moulds  to  the  plank- 
ing which  gives  form  to  the  wall. 

More  elaborate  designs,  especially  if  they  are  of  bold  relief, 
like  cornices,  and  hoods  for  windows  and  doors,  had  better  be 
moulded  in  detached  pieces  some  days  in  advance,  and  hoisted 
into  position  when  required. 

181.  All  kinds  of  masonry  in  thin  walls,  whether  of  brick, 
stone,  common  concrete,  or  beton  agglomere,  are  liable  to 
crack  from  unequal  settlement,  or  from  the  expansion  and 
contraction  due  to  ordinary  changes  of  temperature.  In 


CONCRETE. 


n 


houses,  such  cracks  are  more  to  be  apprehended  at  the  re-en- 
tering angles  of  the  exterior  walls,  and  at  the  junctions  of  the 
exterior  and  partition  walls,  than  elsewhere.  In  concrete  or 
beton  masonry  such  cracks  may  be  prevented  in  a  great 
measure,  without  inconvenience  and  at  a  nominal  cost,  by  em- 
bedding and  incorporating  in  the  work  as  it  progresses, 
at  the  angles  and  junctions  referred  to,  pieces  of  old  scrap- 
iron  of  irregular  shape,  such  as  bolts,  rings,  hooks,  clamps, 
wire,  etc. 

Any  masonry  of  fair  quality,  constructed  in  large  masses 
with  special  reference  to  inertia,  whether  to  resist  the  thrusts 
of  earthen  embankments,  the  statical  pressure  of  water,  the 
force  of  the  current  in  running  streams,  or  for  any  other  pur- 
pose, possesses  a  degree  of  ultimate  strength  much  greater 
than  the  usual  factor  of  safety  would  require,  and  largely  in 
excess  of  any  strain  that  it  would  ever  have  to  sustain.  This 
excess  of  strength,  or  rather  the  material  which  confers  it,  may 
be  readily  saved  in  works  built  of  beton  agglomere,  by  leaving 
large  hollows  or  voids  in  the  heart  of  the  wall,  and  filling  them 
up  with  sand  or  heavy  earth. 

Even  if  the  voids  remain  unfilled,  a  hollow  wall  is  more 
stable  than  a  solid  one  containing  the  same  quantity  of  ma- 
terial, for  the  reason  that  the  moments  of  the  forces  which 
confer  stability  are  greater  in  the  former  than  in  the  latter. 

182.  Durability.  The  densest  mortars  that  can  be  pro- 
duced from  given  materials  are  the  best,  and  the  use  of  a 
large  amount  of  water  is  incompatible  with  the  condition  of 
density. 

The  best  pointing  mortaf,  indeed,  is  a  beton  agglomere,  an- 
swering fully  to  the  description  of  that  material,  being  pre- 
pared with  a  small  proportion  of  water,  and  applied  by  caulk- 
ing it  into  the  joints.  In  northern  climates  it  has  to  sustain 
the  severest  tests  to  which  masonry  of  any  description  can  be 
exposed ;  to  alternations  of  cold  and  heat,  moisture  and  dry- 
ness, freezing  and  thawing. 

Beton  agglomere,  when  the  volume  of  matrix  is  so  adjsuted 
that  the  voids  in  the  sand  are  completely  filled — say  in  the 
roportion  generally  of  one  of  the  matrix  to  two  and  a 
alf  or  three  of  sand — becomes  in  process  of  ti.me  as  imper- 
vious to  water  as  many  of  the  compact  natural  stones,  while 
its  matured  strength  exceeds  that  of  the  best  qualities  of 
sandstone,  some  of  the  granites,  and  many  of  the  limestones 
and  marbles. 

Chemical  tests  have  shown  this  beton  to  be  practically  im- 
pervious to  water. 


72 


CIVIL  ENGINEERING. 


This  material,  therefore,  possesses  all  the  characteristic 
properties  of  durability,  being  dense,  hard,  strong,  and  homo- 
geneous ;  and  there  would  appear  to  be  no  reason  for  suppos- 
ing that  it  may  not,  with  entire  safety,  be  applied  to  out-door 
constructions,  even  in  the  most  northerly  portions  of  the 
United  States. 

It  is  injured  by  freezing  before  it  has  had  time  to  set.  Im- 
portant w^oi'ks  should  not,  therefore,  be  executed  during  the 
winter  in  cold  climates. 

The  effect  of  freezing  on  newly  made  beton  is  to  detach  a 
thin  scale  from  the  exposed  surface,  producing  a  rough  and 
unsightly  appearance  ;  but  the  injury  does  not  extend  into  the 
mass  of  the  material,  unless  the  frost  be  very  intense. 

In  monolithic  constructions,  the  plank  coffre  affords  suffi- 
cient protection  to  the  face  surfaces  of  the  work  against  mod- 
erate frost,  and,  when  the  temperature  ranges  generally  not 
much  lower  than  the  freezing  point  during  the  day,  work 
may  be  safely  carried  on,  if  care  be  taken  to  cover  over  the 
new  material  at  night.  After  it  has  once  set,  and  has  had  a 
few  hours  to  harden,  neither  severe  frost,  nor  alternate  freez- 
ing and  thawing,  has  any  perceptible  effect  upon  it,  and, 
under  any  and  all  circumstances,  it  is  much  less  liable  to 
injury  from  these  causes,  and  requires  fewer  precautions 
for  its  protection  against  them,  than  common  hydraulic  con- 
crete. 

Monolithic  constructions  in  beton  agglomere  may  advan- 
tageously be  carried  on  whenever  it  is  not  too  cold  to  lay  first- 
class  brick  masonry. 

In  Paris  and  vicinity  operations  are  not  generally  suspended 
during  the  winter,  unless  the  cold  be  unusually  severe  for  that 
climate. 

Pieces  of  statuary,  and  other  specimens  ornamented  with 
delicate  tracery,  have  been  exposed  for  five  consecutive  winters 
to  the  weather  in  ISew  York  City,  without  undergoing  the 
slightest  perceptible  change. 

The  power  possessed  by  beton  agglomere  of  resisting  the 
solvent  action  of  salts  (principally  the  sulphates  of  magnesia 
and  soda)  and  certain  gases  contained  in  sea  water,  rests  upon 
analogy  rather  than  upon  proof  based  upon  adequate  experi- 
ence and  observation. 

Eminent  European  engineers  do  not  hesitate  to  use  Portland 
cement  concrete,  mixed  with  a  comparatively  large  dose  of 
water,  for  very  important  submarine  constructions.  The 
matrix  of  this  concrete  possesses  less  density  and  strength 
than  that  of  beton  agglomere,  and  if  the  lime  be  excluded 


CONCRETE. 


73 


from  the  latter,  the  induration  in  the  two  cases  would  be  due 
to  precisely  the  same  chemical  action.  The  materials  are 
indeed  identical  in  composition  under  this  condition,  with  the 
exception  that  there  is  an  excess  of  water,  and  consequently 
an  element  of  weakness,  in  the  English  concrete,  which  does 
not  attach  to  the  beton.  The  durability  of  the  latter  in  sea 
water,  without  being  much  discussed,  has  been  very  generally 
conceded. 

Monolithic  constructions  under  water  cannot  be  executed 
in  beton  agglomere,  for  the  reason  that  the  prescribed  ram- 
ming in  thin  layers  would  necessarily  have  to  be  omitted,  and 
some  other  mode  of  compacting  the  mixture  followed.  This 
material,  however,  when  laid  green  through  water,  loses  its 
distinct  name  and  character,  as  well  as  its  superior  strength 
and  hardness,  and  becomes  common  beton  or  concrete,  with 
the  coarser  ballast  omitted.  Its  use  in  this  form  certainly 
offers  no  advantages  with  regard  to  strength,  while  in  point 
of  economy  the  usual  proportions  of  matrix,  sand  and  shingle, 
or  broken  stone,  is  preferable. 

183.  Adherence  of  Mortar.  The  force  with  which  mor- 
tars in  general  adhere  to  other  materials,  depends  on  the 
nature  of  the  material,  its  texture,  and  the  state  of  the  sur- 
face to  which  the  mortar  is  applied. 

184.  Mortar  adheres  most  strongly  to  brick ;  and  more 
feebly  to  wood  than  to  any  other  material.  Among  stones, 
its  adhesion  to  limestone  is  generally  greatest ;  and  to  basalt 
and  sandstones,  least.  Among  stones  of  the  same  class,  it 
adheres  generally  better  to  the  porous  and  coarse-grained, 
than  to  the  compact  and  fine-grained.  Among  surfaces,  it 
adheres  more  strongly  to  the  rough  than  to  the  smooth. 

185.  The  adhesion  of  common  mortar  to  brick  and  stone, 
for  the  first  few  years,  is  greater  than  the  cohesion  of  its  own 
particles.  The  force  with  which  hydraulic  cement  adheres 
to  the  same  materials,  is  less  than  that  of  the  cohesion  be- 
tween its  own  particles  ;  and,  from  some  recent  experiments 
of  Colonel  Pasley,  on  this  subject,  it  would  seem  that  hy- 
draulic cement  adheres  with  nearly  the  same  force  to  polished 
surfaces  of  stone  as  to  rough  surfaces. 

186.  From  experiments  made  by  Kondelet,  on  the  adhesion 
of  common  mortar  to  stone,  it  appears  that  it  required  a  force 
varying  from  15  to  30  pounds  on  the  square  inch,  applied 
perpendicular  to  the  plane  of  the  joint,  to  separate  the  mortar 
and  stone  after  six  months  union ;  whereas  only  5  pounds  to 
the  square  inch  was  required  to  separate  the  same  surfaces, 
when  applied  parallel  to  the  plane  of  the  joint. 


CIVIL  ENGINEERING. 


From  experiments  made  by  Colonel  Paslej,  he  concludes 
that  the  adhesive  force  of  hydraulic  cement  to  stone,  may  be 
taken  as  high  as  125  pounds  on  the  square  inch,  when  the 
joint  has  had  time  to  harden  throughout;  but,  he  remarks, 
that  as  in  large  joints  the  exterior  part  of  the  joint  may  have 
hardened  while  the  interior  still  remains  soft,  it  is  not  safe  to 
estimate  the  adhesive  force,  in  such  cases,  higher  than  from 
30  to  40  pounds  on  the  square  inch. 


YL 

MASTICS. 


187.  The  term  Mastic  is  generally  applied  to  artificial  or 
natural  combinations  of  bituminous  or  resinous  substances 
with  other  ingredients.  They  are  converted  to  various  uses 
in  constructions,  either  as  cements  for  other  materials,  or  as 
coatings,  to  render  them  imper^dous  to  water. 

188.  Bituminous  Mastic.  The  knowledge  of  this  ma- 
terial dates  back  to  an  early  period ;  but  it  is  only  within, 
comparatively  speaking,  a  few  years  that  it  has  come  into 
common  use  in  Europe  and  this  country.  The  most  usual 
form  in  which  it  is  now  employed,  is  a  combination  of  min- 
eral tar  and  powdered  bituminous  limestone. 

189.  The  localities  of  each  of  these  substances  are  very 
numerous ;  but  they  are  chiefly  brought  into  the  market  from 
several  places  in  Switzerland  and  France,  where  these  min- 
erals are  found  in  great  abundance  ;  the  most  noted  being 
Yal-de-Travers  in  Switzerland,  and  Se^-ssel  in  France. 

190.  The  mineral  tar  is  usually  obtained  by  boiling  in 
water  a  soft  sandstone,  called  by  the  French  molasse,  which 
is  strongly  impregnated  with  the  tar.  In  this  process,  the  tar 
is  disengaged  and  rises  to  the  surface  of  the  water,  or  adheres 
to  the  sides  of  the  vessel,  and  the  earthy  matter  remains  at 
the  bottom.  An  analysis  of  a  rich  specimen  of  the  Seyssel 
bituminous  sandstone  ffave  the  folio  wine*  results : — 


Bitumen  106 


Bituminous  oil  086 

Carbon  020 

Quartzy  grains  690 

Calcareous  grains  204 


1.000 


MASTICS. 


75 


191.  The  bituminous  limestone  which,  when  reduced  to  a 
powdered  state,  is  mixed  with  the  mineral  tar,  is  known  at  the 
localities  mentioned  by  the  name  of  asphaltum^  an  appellation 
which  is  now  usually  given  to  the  mastic.  This  limestone  oc- 
curs in  the  secondary  formations,  and  is  found  to  contain 
various  proportions  of  bitumen,  varying  mostly  from  3  to  15 
per  cent.,  with  the  other  ordinary  minerals,  as  argile,  etc., 
which  are  met  with  in  this  formation. 

192.  The  clay  contained  in  asphaltic  rock,  as  it  is  not  im- 
pregnated, like  the  carbonate  of  lime,  with  the  bitumen,  is 
hurtful,  causing,  at  times,  the  cracks  seen  in  asphaltic  pave- 
ments. 

Some  rocks  contain  an  oily  element,  like  petroleum,  which, 
rendering  the  mastic  made  from  them  too  fat,  must  first  be 
distilled  out. 

193.  The  bituminous  mastic  is  prepared  from  these  two 
materials  by  heating  the  mineral  tar  in  cast-iron  or  sheet-iron 
boilers,  and  stirring  in  the  proper  proportion  of  the  powdered 
limestone.  This  operation,  although  very  simple  in  its  kind, 
requires  great  attention  and  skill  on  the  part  of  the  workmen 
in  managing  the  fire,  as  the  mastic  maybe  injured  by  too  low, 
or  too  high  a  degree  of  heat.  The  best  plan  appears  to  be,  to 
apply  a  brisk  fire  until  the  boiling  liquid  commences  to  give 
out  a  thin  whitish  vapor.  The  fire  is  then  moderated  and 
kept  at  a  uniform  state,  and  the  powdered  stone  is  gradually 
added,  and  mixed  in  with  the  tar  by  stirring  the  two  well  to- 
gether. When  the  temperature  has  been  raised  too  high,  the 
heated  mass  gives  out  a  yellowish  or  brownish  vapor.  In  this 
state  it  should  be  stirred  rapidly,  and  be  removed  at  once  from 
the  fire. 

194.  The  asphaltic  stone  may  be  reduced  to  powder,  either 
by  roasting  it  in  vessels  over  a  fire,  or  by  grinding  it  down  in 
the  ordinary  mortar-mill.  For  roasting,  the  stone  is  first  re- 
duced to  fragments  the  size  of  an  egg.  These  fragments  are 
put  into  an  iron  vessel ;  heat  is  applied,  and  the  stone  is  re- 
duced to  powder  by  stirring  it  and  breaking  it  up  with  an 
iron  instrument.  This  process  is  not  only  less  economical  than 
grinding,  but  the  material  loses  a  portion  of  its  tar  from 
evaporation,  besides  being  liable  to  injury  from  too  great  a 
degree  of  heat.  For  grinding,  the  stone  is  first  broken  as  for 
roasting.  Care  should  be  taken,  during  the  process,  to  stir  the 
mass  frequently,  otherwise  it  may  form  into  a  cake.  Cold  dry 
weather  is  the  best  season  for  this  operation ;  the  stone,  how- 
ever, should  not  be  exposed  to  the  weather. 

195.  Owing  to  the  variable  quantity  of  mineral  tar  in 


76 


CIVIL  ENGrNEERING. 


bituminous  limestone,  the  best  proportions  of  the  tar  and 
powdered  stone  for  bituminous  mastic  cannot  be  assigned  be- 
forehand. Three  or  four  per  cent,  too  much  of  tar  is  said  to 
impair  both  the  durability  and  tenacity  of  the  mastic ;  while 
too  small  a  quantity  is  equally  prejudicial.  Generally,  from 
eight  to  ten  per  cent,  of  the  tar,  by  weight,  has  been  found  to 
yield  a  favorable  result. 

196.  Mastics  have  been  formed  by  mixing  vegetable  tar, 
pitch,  and  other  resinous  substances,  with  litharge,  powdered 
brick,  powdered  limestone,  etc. ;  but  the  results  obtained  have 
generally  been  inferior  to  those  from  bituminous  mastic. 

197.  Mineral  tar  is  more  durable  than  vegetable  tar,  and  on 
this  account  it  has  been  used  alone  as  a  coating  for  other 
materials,  but  not  with  the  same  success  as  mastic.  Employed 
in  this  way  the  tar  in  time  becomes  dry  and  peels  off ;  where- 
as, in  the  form  of  mastic,  the  hard  matter  with  which  it  is 
mixed  prevents  the  evaporation  of  the  oily  portion  of  the  tar, 
and  thus  promotes  its  durability. 

198.  The  uses  to  which  bituminous  mastic  is  applied  are 
daily  increasing.  It  has  been  used  for  paving  in  a  variety  of 
forms  either  as  a  cement  for  large  blocks  of  stone,  or  as  the 
matrix  of  a  concrete  formed  of  small  fragments  of  stone  or 
gravel ;  as  a  pointing,  it  is  found  to  be  more  serviceable,  for 
some  purposes,  than  hydraulic  cement ;  it  forms  one  of  the  best 
water-tight  coatings  for  cisterns,  cellars,  the  cappings  of  arches, 
terraces,  and  other  similar  roofings  now  in  use  ;  and  is  a  good 
preservative  agent  for  wood-work  exposed  to  wet  or  damp. 


YII. 

BRICK. 

199.  This  material  is  properl}^  an  artificial  stone,  formed  by 
submitting  common  clay,  which  has  undergone  suitable  pre- 
paration, to  a  temperature  sufficient  to  convert  it  into  a  semi- 
vitrified  state. 

Brick  may  be  used  for  nearly  all  the  purposes  to  which 
stone  is  applicable  ;  for  when  carefully  made,  its  strength,  hard- 
ness, and  durability,  are  but  little  inferior  to  the  more  ordinary 
kinds  of  buildiufj  stone.  It  remains  unchano^ed  under  the  ex- 
tremes  of  temperature ;  resists  the  action  of  water ;  sets  nj-mly 


BEICK8.  77 

and  promptly  with  mortar;  and  being  both  cheaper  and 
lighter  than  stone,  is  preferable  to  it  for  many  kinds  of  struc- 
tures, as  arches,  the  walls  of  houses,  &c, 

200.  The  art  of  brick-making  is  a  distinct  branch  of  the 
useful  arts,  and  does  not  properly  belong  to  that  of  the  en- 
gineer. But  as  the  engineer  may  at  times  be  obliged  to  pre- 
pare this  material  himself,  the  following  outline  of  the  process 
may  prove  of  service. 

201.  The  best  brick  earth  is  composed  of  a  mixture  of  pure 
clay  and  sand,  deprived  of  pebbles  of  every  kind,  but  par- 
ticularly of  those  which  contain  lime,  and  pyritous  or  other 
metallic  substances;  as  these  substances,  when  in  large 
quantities,  and  in  the  form  of  pebbles,  act  as  fluxes,  and  de- 
stroy the  shape  of  the  brick,  and  weaken  it  by  causing  cavities 
and  cracks ;  but  in  small  quantities,  and  equally  diffused 
throughout  the  earth,  they  assist  the  vitrification,  and  give  it 
a  more  uniform  character. 

202.  Good  brick  earth  is  frequently  found  in  a  natural 
state,  and  requires  no  other  preparation  for  the  purposes  of 
the  brick-maker.  When  he  is  obliged  to  prepare  the  earth  by 
mixing  the  pure  clay  and  sand,  direct  experiments  should  in 
all  cases  be  made,  to  ascertain  the  proper  proportions  of  the 
two.  If  the  clay  is  in  excess,  the  temperature  required  to 
semi-vitrify  it  will  cause  it  to  warp,  shrink,  and  crack ;  and 
if  there  is  an  excess  of  sand,  complete  vitrification  will  ensue, 
under  similar  circumstances. 

203.  The  quality  of  the  brick  depends  as  much  on  the 
care  bestowed  on  its  manufacture,  as  on  the  quality  of  the 
earth.  The  first  stage  of  the  process  is  to  free  the  earth  from 
pebbles,  which  is  most  effectually  done  by  digging  it  out  early 
in  the  autumn,  and  exposing  it  in  small  heaps  to  the  weather 
during  the  winter.  In  the  spring  the  heaps  are  carefully 
riddled,  if  necessary,  and  the  earth  is  then  in  a  proper  state 
to  be  kneaded  or  tempered.  The  quantity  of  water  required 
in  tempering  will  depend  on  the  quality  of  the  earth  ;  no 
more  should  be  used  than  will  be  sufficient  to  make  the  earth 
so  plastic  as  to  admit  of  its  being  easily  moulded  by  the 
workman.  About  half  a  cubic  foot  of  water  to  one  of  the 
earth  is,  in  most  cases,  a  good  proportion.  If  too  much  water 
be  used,  the  brick  will  not  only  be  very  slow  in  drying,  but 
it  will,  in  most  cases,  crack,  owing  to  the  surface  becoming 
completely  dry  before  the  moisture  of  the  interior  has  had 
time  to  escape  ;  the  consequence  of  which  will  be,  that  the 
brick,  when  burnt,  will  be  either  entirely  unfit  for  use,  or  very 
weak. 


78 


CIVIL  ENGINEERING. 


204.  Machinery  is  now  coming  into  very  general  use  in 
moulding  brick :  it  is  superior  to  manual  labor,  not  only 
from  the  labor  saved,  but  from  its  yielding  a  better  quality 
of  brick,  by  giving  it  great  density,  which  adds  to  its 
strength. 

205.  Great  attention  is  requisite  in  drying  the  brick  before 
it  is  burned.  It  should  be  placed,  for  this  purpose,  in  a  dry 
exposure,  and  be  sheltered  from  the  direct  action  of  the  wind 
and  sun,  in  order  that  the  moisture  may  be  carried  off  slowly 
and  uniformly  from  the  entire  surface.  When  this  precau- 
tion is  not  taken,  the  brick  will  generally  crack  from  the  un- 
equal shrinking,  arising  from  one  part  drying  more  rapidly 
than  the  rest. 

206.  The  burning  and  cooling  should  be  done  with  equal 
care.  A  very  moderate  fire  should  be  applied  under  the  arches 
of  the  kiln  for  about  twenty-four  hours,  to  expel  any  remain- 
ing moisture  from  the  raw  brick ;  this  is  known  to  be  com- 
pletely effected  when  the  smoke  from  the  kiln  is  no  longer 
black.  The  fire  is  then  increased  until  the  bricks  of  the 
arches  attain  a  white  heat ;  it  is  then  allowed  to  abate  in 
some  degree,  in  order  to  prevent  complete  vitrification ;  and 
it  is  alternately  raised  and  lowered  in  this  way  until  the 
burning  is  comj^lete,  which  may  be  ascertained  by  examining 
the  bricks  at  the  top  of  the  kiln.  The  cooling  should  be 
slowl}^  effected ;  otherwise  the  bricks  will  not  withstand  the 
effects  of  the  weather.  It  is  done  by  closing  the  mouths  of 
the  arches,  and  the  top  and  sides  of  the  kihi,  in  the  most  ef- 
fectual manner  with  moist  clay  and  burnt  brick,  and  allow- 
ing the  kiln  to  remain  in  this  state  until  the  warmth  has  sub- 
sided. 

207.  Brick  of  a  good  quality  exhibits  a  fine,  compact,  uni- 
form texture,  when  broken  across  ;  gives  a  clear,  ringing 
sound,  when  struck ;  and  is  of  a  cherry  red,  or  brownish 
color.  Three  varieties  are  found  in  the  kiln :  those  which 
form  the  arches,  denominated  arch  hricJc^  are  always  vitrified 
in  part,  and  present  a  grayish  glassy  appearance  at  one  end  ; 
they  are  very  hard,  but  brittle,  of  inferior  strength,  and  set 
badly  with  mortar;  those  from  the  interior  of  the  kiln, 
usually  denominated  hody^  hard^  or  cJierry  hrich,  are  of  the 
best  qiialit}^ ;  those  from  near  the  top  and  sides  are  generally 
underburnt,  and  are  denominated  soft,  pale,  or  sammel 
hriclc  ;  they  have  neither  sufficient  strength  nor  durability  for 
heavy  masonry,  nor  the  outside  courses  of  walls  which  are 
exposed  to  the  weather. 

208.  The  quality  of  good  brick  may  be  improved  by  soak- 


TIMBER. 


79 


ing  it  for  some  days  in  water,  and  re-burning  it.  This  pro- 
cess increases  both  the  strength  and  durability,  and  renders 
the  brick  more  suitable  for  hydraulic  constructions,  as  it  is 
found  not  to  imbibe  water  so  readily  after  having  under- 
gone it. 

209.  The  size  and  form  of  bricks  present  but  trifling  varia- 
tions. They  are  generally  rectangular  parallelopipeds,  from 
eight  to  nine  inches  long,  from  four  to  four  and  a  half  wide, 
and  from  two  to  two  and  a  quarter  thick.  Thin  brick  ia 
generally  of  a  better  quality  than  thick,  because  it  can  be 
dried  and  burned  more  uniformly. 

210.  Fire-brick.  This  material  is  used  for  the  facing  of 
furnaces,  fireplaces,  &c.,  where  a  high  degree  of  temperature 
is  to  be  sustained.  It  is  made  of  a  very  refractory  kind  of 
pure  clay,  that  remains  unchanged  by  a  degree  of  heat  which 
would  vitrify  and  completely  destroy  ordinary  brick.  A 
very  remarkable  brick  of  this  character  has  been  made  of 
agaric  mineral  j  it  remains  unchanged  under  the  highest 
temperature,  is  one  of  the  worst  conductors  of  heat,  and  so 
light  that  it  will  float  on  water. 

211.  Tiles.  As  a  roof-covering,  tiles  are  in  many  respects 
superior  to  slate,  or  metallic  coverings.  They  are  strong  and 
durable,  and  are  very  suitable  for  the  covering  of  arches,  as 
their  great  weight  is  not  so  objectionable  here  as  in  the  case 
of  roofs  formed  of  frames  of  timber. 

Tiles  should  be  made  of  the  best  potter's  clay,  and  be 
moulded  with  great  care,  to  give  them  the  greatest  density 
and  strength.  They  are  of  very  variable  form  and  size  ;  the 
worst  being  the  flat  square  form,  as,  from  the  liability  of  the 
clay  to  warp  in  burning,  they  do  not  make  a  perfectly  water- 
tight covering. 


YIIL 

WOOD. 

212.  This  material  holds  the  next  rank  to  stone,  owing  to 
its  durability  and  strength,  and  the  very  general  use  made  of 
it  in  constructions.  To  suit  it  to  the  purposes  of  the  en- 
gineer, the  tree  is  felled  after  having  attained  its  mature 
growth,  and  the  trunk,  the  larger  branches  that  spring  from 
the  trunkj  and  the  main  parts  of  the  root,  are  cut  into  suita^ 


80 


CIVIL  ENGINEEEtXa. 


ble  dimensions  and  seasoned,  in  which  state  the  term  tim- 
her  is  applied  to  it.  The  crooked,  or  compass  timber  of  the 
branches  and  roots  is  mostl}^  applied  to  the  purposes  of  ship- 
building— for  the  knees  and  other  parts  of  the  frame- work  of 
vessels  requiring  crooked  timber.  The  trunk  furnishes  all 
-the  straight  timber. 

213.  Trunk.  The  trunk  of  a  full-grown  tree  presents 
three  distinct  parts :  the  harJc,  which  forms  the  extei-ior  coat- 
ing ;  the  sap-wood^  which  is  next  to  the  bark ;  the  hearty  or 
inner  part,  which  is  easily  distinguisliable  from  the  sap-wood 
by  its  greater  firmness  and  darker  color. 

214.  The  heart  forms  the  essential  part  of  the  trunk,  as  a 
building  material.  The  sap-wood  possesses  but  little  strength 
and  is  subject  to  rapid  decay,  owing  to  the  great  quantity  of 
fermentable  matter  contained  in  it ;  and  the  bark  is  not  only 
without  strength,  but,  if  suffered  to  remain  on  the  tree  after 
it  is  felled,  it  hastens  the  decay  of  the  sap-wood  and  heart. 

215.  Felling.  Trees  should  not  be  felled  for  timber  until 
they  have  attained  their  mature  growth,  nor  after  they  exhibit 
symptoms  of  decline  ;  otherwise,  the  timber  will  be  less 
strong,  and  far  less  durable.  Most  forest  trees  arrive  at  ma- 
turity between  fifty  and  one  hundred  years,  and  commence 
to  decline  after  one  hundred  and  fifty  or  two  hundred  years. 
The  age  of  the  tree  can,  in  most  cases,  be  ascertained  either 
by  its  external  appearances,  or  by  cutting  into  the  centre  of 
the  trunk,  and  counting  the  rings,  or  layers,  of  the  sap  and 
heart,  as  a  new  ring  is  formed  each  year  in  the  process  of 
vegetation.  When  the  tree  commences  to  decline,  the  ex- 
tremities of  the  old  branches,  and  particularly  the  toj),  exhibit 
signs  of  decay. 

216.  Trees  should  not  be  felled  while  the  sap  is  in  circula- 
tion ;  for  this  substance  is  of  a  peculiarly  fermentable  nature, 
and  therefore  very  productive  of  destruction  to  the  wood. 
The  winter  months,  and  July,  are  the  seasons  in  which  trees 
are  felled  for  timber,  as  the  sap  is  generally  considered  as 
dormant  during  these  months.  This  practice,  however,  is  in 
part  condemned  by  some  writers ;  and  the  recent  experiments 
of  M.  Boucherie,  in  France,  support  this  opinion,  and  indicate 
midsummer  and  autumn  as  the  seasons  in  which  the  sap  is 
least  active,  and  therefore  as  most  favorable  for  felling. 

217.  Girdling  and  Barking.  As  the  sap-wood,  in  most 
trees,  forms  a  large  portion  of  the  trunk,  experiments  have 
been  made  for  the  purpose  of  improving  its  strength  and 
durability.  These  experiments  have  been  mostly  directed 
towards  the  manner  of  preparing  the  tree  before  felling  it. 


TIMBER. 


81 


One  method  consists  in  girdling,  or  making  an  incision  with 
an  axe  around  the  trunk,  completely  through  the  sap-wood, 
and  suffering  the  tree  to  stand  in  this  state  until  it  is  dead ; 
the  other  consists  in  'barking,  or  stripping  the  entire  trunk  of 
its  bark,  without  wounding  the  sap-wood,  early  in  the  spring, 
and  allowing  the  tree  to  stand  until  the  new  leaves  have  put 
forth  and  fallen  before  it  is  felled.  The  sap-wood  of  trees, 
treated  by  both  of  these  methods,  was  found  very  much 
improved  in  hardness,  strength,  and  durability ;  the  results 
from  girdling  were,  however,  inferior  to  those  from  barking. 

218.  Methods  of  Seasoning.  The  seasoning  of  timber  is 
of  the  greatest  importance,  not  only  to  its  durability,  but  to 
the  solidity  of  the  structure  for  which  it  may  be  nsed ;  as  a 
very  slight  shrinking  of  some  of  the  pieces,  arising  from  the 
seasoning  of  the  wood,  might,  in  many  cases,  cause  material 
injury,  if  not  complete  destruction  to  the  structure.  Timber 
is  considered  as  sufficiently  seasoned,  for  the  purposes  of  frame- 
work, wdien  it  has  lost  about  one-fifth  of  the  weight  which  it 
has  in  a  green  state.  Several  methods  are  in  use  for  season- 
ing timber :  they  consist  either  in  an  exposure  to  the  air  for  a 
certain  period  in  a  sheltered  position,  which  is  termed  natu- 
ral seasoning  /  in  immersion  in  water,  termed  water  season- 
ing y  or  in  boiling,  or  steaming. 

219.  For  natural  seasoning,  it  is  nsually  recommended  to 
strip  the  trunk  of  its  branches  and  bark  immediately  upon, 
felling,  and  to  remove  it  to  some  dry  position,  until  it  can  be- 
sawed  into  suitable  .scantling.  From  the  experiments  of  M, 
Boucherie,  just  cited,  it  wTmld  seem  that  better  results  would, 
ensue  from  allowing  the  branches  and  bark  to  remain  on  the- 
trunk  for  some  days  after  felling.  In  this  state,  the  vital  ac- 
tion of  the  tree  continuing  in  operation,  the  sap-vessels  will  be 
gradually  exhausted  of  sap  and  filled  with  air,  and  the  trunk 
thus  better  prepared  for  the  process  of  seasoning.  To  com- 
plete the  seasoning,  the  sawed  timber  should  be  piled  under 
drying-sheds,  where  it  will  be  freely  exposed  to  the  circula- 
tion of  the  air,  but  sheltered  from  the  direct  action  of  the  wind, 
rain,  and  sun.  By  taking  these  precautions,  an  equable  eva- 
poration of  the  moisture  will  take  place  over  the  entire  sur- 
face, which  will  prevent  either  warping  or  splitting,  w^hich 
necessarily  ensues  when  one  part  dries  more  rapidly  than  an- 
other. It  is  further  recommended,  instead  of  piling  the 
pieces  on  each  other  in  a  horizontal  position,  that  they  be  laid; 
on  cast-iron  supports  properly  prepared,  and  with  a  sufficient 
inclination  to  facilitate  the  dripping  of  the  sap  from  one  end  ; 
and  that  heavy  round  timber  be  bored  through  the  centre,  to 

6 


82 


CIVIL  ENGINEERING. 


expose  a  greater  surface  to  the  air,  as  it  has  been  found  that 
it  cracks  more  in  seasoning  than  square  timber. 

Natural  seasoning  is  preferable  to  any  other,  as  timber  sea- 
soned in  this  way  is  both  stronger  and  more  durable  than  when 
prepared  by  any  artificial  process.  Most  timber  will  require, 
on  an  average,  about  two  years  to  become  fully  seasoned  in 
the  natural  way. 

220.  The  process  of  seasoning  by  immersion  in  water  is 
slow  and  imperfect,  as  it  takes  years  to  saturate  heavy  timber ; 
and  the  soluble  matter  is  discharged  very  slowly,  and  chiefly 
from  the  exterior  layers  of  the  immersed  wood.  The  practice 
of  keeping  timber  in  water,  with  a  view  to  facilitate  its  sea- 
soning, has  been  condemned  as  of  doubtful  utility ;  particu- 
larly immersion  in  salt  water,  where  the  timber  is  liable  to  the 
inroads  of  those  two  very  destructive  inhabitants  of  our  waters, 
the  Limnoria  Terebrans  and  Teredo  Navalis  ^  the  former 
of  which  rapidly  destroys  the  heaviest  logs,  by  gradually  eat- 
ing in  between  the  annual  rings  ;  and  the  latter,  the  well- 
known  sliijp-worra^  by  converting  timber  into  a  perfect  honey- 
comb state  by  its  numerous  perforations. 

221.  Steaming  is  mostly  in  use  for  ship-building,  where  it 
is  necessary  to  soften  the  fibres,  for  the  purpose  of  bending 
large  pieces  of  timber.  This  is  effected  by  placing  the  timber 
in  strong  steam-tight  cylinders,  where  it  is  subjected  to  the 
action  of  steam  long  enough  for  the  object  in  view ;  the 
period  usually  allowed  is  one  hour  to  each  inch  in  thickness. 
Steaming  slightly  impairs  the  strength  of*timber,  but  renders 
it  less  subject  to  decay,  and  less  liable  to  warp  and  crack. 

222.  When  timber  is  used  for  posts  partly  embedded  in  the 
ground,  it  is  usual  to  char  the  part  embedded,  to  preserve  it 
irom  decay.  This  method  is  only  serviceable  when  the  timber 
has  been  previously  well  seasoned ;  but  for  green  timber  it  is 
highly  injurious,  as  by  closing  the  pores  it  prevents  the  evap- 
oration from  the  surface,  and  thus  causes  fermentation  and 
rapid  decay  within. 

223.  The  most  durable  timber  is  procured  from  trees  of  a 
close,  compact  texture,  which,  on  analysis,  yield  the  largest 
quantity  of  carbon.  And  those  which  grow  in  moist  and 
shady  localities  furnish  timber  which  is  weaker  and  less  dur- 
able than  that  from  trees  growing  in  a  dry,  open  exposure. 

224.  Defects  of  Timber.  Timber  is  subject  to  defects, 
arising  either  from  some  peculiarity  in  the  growth  of  the  tree, 
or  from  the  effects  of  the  weather.  Straight-grained  timber, 
free  from  knots,  is  superior  in  strength  and  quality  as  a  build- 
ing material  to  that  which  is  the  reverse. 


TIMBER. 


83 


225.  The  action  of  high  winds,  or  of  severe  frosts,  injures 
the  tree  while  standing :  the  former  separating  the  layers  from 
each  other,  forming  what  is  denominated  rolled  timber  /  the 
latter  cracking  the  timber  in  several  places,  from  the  surface 
to  the  centre.  These  defects,  as  well  as  those  arising  from 
w^orms,  or  age,  are  easily  seen  b}^  examining  a  cross  section  of 
the  trunk. 

226.  Wet  and  Dry  Rot.    The  wet  and  dry  rot  are  the 

most  serious  causes  of  the  decay  of  timber ;  as  all  the  remedies 
thus  far  proposed  to  prevent  them  are  too  expensive  to  admit 
of  a  very  general  application.  Both  of  these  causes  have  the 
same  origin :  fermentation,  and  consequent  putrefaction.  The 
wet  rot  takes  place  in  wood  exposed,  alternately,  to  moisture 
and  dryness  ;  and  the  dry  rot  is  occasioned  by  want  of  a  free 
circulation  of  air,  as  in  confined  warm  localities,  like  cellars 
and  the  more  confined  parts  of  vessels. 

Trees  of  rapid  growth,  which  contain  a  large  portion  of 
sap-wood,  and  timber  of  every  description,  when  used  green, 
where  there  is  a  want  of  a  free  circulation  of  air,  decay  very 
rapidly  with  the  rot. 

227.  Preservation  of  Timber.  Numberless  experiments 
have  been  made  on  the  preservation  of  timber,  and  many 
processes  for  this  purpose  have  been  patented,  both  in  Europe 
and  this  country.  Several  of  these  processes  have  yielded 
the  most  satisfactory  results ;  and  nearly  all  have  proved 
more  or  less  efficacious.  The  means  mostly  resorted  to  have 
been  the  saturation  of  the  timber  in  the  solution  of  some  salt 
with  a  metallic  or  earthy  base,  thus  forming  an  insoluble 
compound  with  the  soluble  matter  of  the  timber.  The  salts 
which  have  been  most  generally  tried  are,  the  sulphate  of 
iron  or  copper,  and  the  chloride  of  mercury,  zinc,  or  calcium. 
The  results  obtained  from  the  chlorides  have  been  more  satis- 
factory than  those  from  the  sulphates ;  the  latter  class  of  salts 
with  metallic  bases  possess  undoubted  antiseptic  properties ; 
but  it  is  stated  that  the  freed  sulphuric  acid,  arising  from 
the  chemical  action  of  the  salt  on  the  wood,  impairs  the 
woody  fibre,  and  changes  it  into  a  substance  resembling 
carbon. 

228.  The  processes  which  have  come  into  most  general  use 
are  those  of  Mr.  Kyan  and  of  Sir  W.  Burnett,  called  after 
the  patentees  Jcyanizing  and  hurnetizing.  Kyan's  process  is 
to  saturate  the  timber  with  a  solution  of  chloride  of  mercury ; 
using  for  the  solution  one  pound  of  the  salt  to  five  gallons  of 
water.  Burnett  uses  a  solution  of  chloride  of  zinc,  in  the  pro- 
portion of  one  pound  of  the  salt  to  ten  gallons  of  water,  for 


84: 


CIYIL  ENGINEERING. 


common  purposes ;  and  a  more  tighly  concentrated  solution 
when  the  object  is  also  to  render  the  wood  incombustible. 

229.  As  timber  under  the  ordinary  circumstances  of  im- 
mersion imbibes  the  solutions  very  slowly,  a  more  expeditious, 
as  well  as  more  perfect  means  of  saturation  has  been  used  of 
late,  which  consists  in  placing  the  wood  to  be  prepared  in 
strong  wrouglit-iron  cylinders,  lined  with  felt  and  boards,  to 
protect  the  iron  from  the  action  of  the  solution,  where,  first 
by  exhausting  the  cylinders  of  air,  and  then  applying  a  strong 
pressure  by  means  of  a  force-pump,  the  liquid  is  forced  into 
the  sap  and  air  vessels,  and  penetrates  to  the  very  centre  of 
the  timber. 

230.  Among  the  patented  processes  in  our  country,  that  of 
Mr.  Earle  has  received  most  notice.  This  consists  in  boiling 
the  timber  in  a  solution  of  the  sulphates  of  copper  and  iron. 
Opinion  seems  to  be  divided  as  to  the  efficacy  of  this  method. 
It  has  been  tried  for  the  preservation  of  timber  for  artillery 
carriages,  but  not  with  satisfactory  results. 

231.  M.  Boucherie,  to  whose  able  researches  on  this  subject 
reference  has  been  made,  noticing  the  slowness  with  which 
aqueous  sohitions  were  imbibed  by  wood,  when  simply  im- 
mersed in  them,  conceived  the  ingenious  idea  of  rendering 
the  vital  action  of  the  sap-vessels  subservient  to  a  thorough 
impregnation  of  every  part  of  the  trunk  where  there  was  this 
vitality.  To  effect  this,  he  first  immersed  the  butt-end  of  a 
freshly-felled  tree  in  a  liquid,  and  found  that  it  was  diffused 
throughout  all  parts  of  the  tree  in  a  few  days,  by  the  action 
in  question.  But,  finding  it  difficult  to  manage  trees  of  some 
size  when  felled,  M.  Boticherie  next  attempted  to  saturate 
them  before  felling ;  for  which  purpose  he  bored  an  auger- 
hole  through  the  trunk,  and  made  a  saw-cut  from  the  auger- 
hole  outwards,  on  each  side,  to  within  a  few  inches  of  the 
exterior,  leaving  enough  of  the  fibres  untouched  to  support 
the  tree.  One  end  of  the  auger-hole  was  then  stopped,  as 
well  as  all  of  the  saw-cut  on  the  exterior,  and  the  liquid  was 
introduced  by  a  tube  inserted  into  the  open  end  of  the  auger- 
hole.  This  method  was  found  equally  efficacious  with  the 
first,  and  more  convenient. 

232.  After  examining  the  action  oE  the  various  neutral 
salts  on  the  soluble  matter  contained  in  wood,  M.  Boucherie 
was  led  to  try  the  impure  pyrolignite  of  iron,  both  from  its 
chemical  composition  and  its  cheapness.  The  results  of  this 
experiment  were  perfectly  satisfactory.  The  pyrolignite  of 
iron,'  in  the  proportion  of  one-fiftieth  in  weight  of  the  green 


TIMBER. 


85 


wood,  was  found  not  only  to  preserve  the  wood  from  decay, 
but  to  harden  it  to  a  very  high  degree. 

233.  Observing  that  the  pliability  and  elasticity  of  wood 
depended,  in  a  great  measure,  on  the  moisture  contained  in  it, 
M.  Boucherie  next  directed  his  attention  to  the  means  of 
improving  these  properties.  For  this  purpose  he  tried  solu- 
tions of  various  deliquescent  salts,  which  were  found  to  an- 
swer the  end  proposed.  Among  these  solutions  he  gives  the 
preference  to  that  of  chloride  of  calcium,  which  also,  when 
concentrated,  renders  the  wood  incombustible.  He  also  re- 
commends for  like  purposes  the  mother- water  of  salt-marshes, 
as  cheaper  than  the  solution  of  the  chloride  of  calcium. 
Timber  prepared  in  this  way  is  not  only  improved  in  elasticity 
and  pliability,  but  is  prevented  from  warping  and  cracking  ; 
the  timber,  however,  is  subject  to  greater  variations  in  weight 
than  when  seasoned  naturally. 

234.  M.  Boucherie  is  of  opinion  that  the  earthy  chlorides 
will  also  act  as  preservatives,  but  to  insure  this  he  recom 
mends  that  they  be  mixed  with  one-fifth  of  pyrolignite  of 
iron. 

235.  From  other  experiments  of  M.  Boucherie,  it  appears 
that  the  sap  may  be  expelled  from  any  freshly-felled  timber 
by  the  pressure  of  a  liquid,  and  the  timber  be  impregnated 
as  thoroughly  as  by  the  preceding  processes.  To  effect  this, 
the  piece  to  be  saturated  is  placed  in  an  upright  position,  so 
that  the  sap  may  flow  readily  from  the  lower  end  ;  a  water- 
tight bag,  containing  the  liquid,  is  affixed  to  the  upper  ex- 
tremity, which  is  surmounted  by  the  liquid,  the  pressure  from 
which  expels  the  sap,  and  fills  the  sap-vessels  with  the  liquid. 
The  process  is  complete  when  the  liquid  is  found  to  issue  in  a 
pure  state  from  the  lower  end  of  the  stick. 

237.  Either  of  the  above  processes  may  be  applied  in  im- 
pregnating timber  with  coloring  matter  for  ornamental  pur- 
poses. The  plan  recommended  by  M.  Boucherie  consists  in 
introducing  separately  the  solutions  by  the  chemical  union  of 
which  the  color  is  to  be  formed. 

238.  The  rapid  decay  of  railroad  sleepers  has  led  to  more 
recent  experiments  in  Europe,  where  timber  is  scarce  and 
dear.  Opinion  now  is  in  favor  of  impregnating  them  with 
creosote,  as  the  best  preservative  from  wet  rot. 

239.  The  effect  of  time  on  the  durability  of  timber,  pre- 
pared by  any  of  the  various  chemical  processes  which  have 
just  been  detailed,  remains  to  bB  seen ;  although  results  of 
the  most  satisfactory  nature  may  be  looked  for,  considering 
the  severe  tests  to  which  most  of  them  have  been  submitted, 


86 


CIVIL  ENGESrEERING. 


by  exposure  in  situations  peculiarly  favorable  to  the  destruc- 
tion of  ligneous  substances. 

240.  Durability  of  Timber.  The  durability  of  timber, 
when  not  prepared  by  any  of  the  above-mentioned  processes, 
varies  greatly  under  different  circumstances  of  exposure.  If 
placed  in  a  sheltered  position,  and  exposed  to  a  free  circula- 
tion of  air,  timber  will  last  for  centuries,  without  showing  any 
sensible  changes  in  its  physical  properties.  An  equal,  if  not 
superior,  durabihty  is  observed  when  it  is  immersed  in  fresh 
water,  or  embedded  in  thick  walls,  or  underground,  so  as  to 
be  beyond  the  influence  of  atmospheric  changes. 

241.  In  salt  water,  however,  particularly  in  warm  climates, 
timber  is  rapidly  destroyed  by  the  two  animals  already 
noticed :  the  one,  the  limnoria  terebrans^  attacking,  it  is  said, 
only  stationary  wood,  while  the  attacks  of  the  other,  the 
teredo  navalis,  are  general.  Various  means  have  been  tried 
to  guard  against  the  ravages  of  these  destructive  agents ;  that 
of  sheathing  exposed  timber  with  copper,  or  with  a  coating 
of  hydraulic  cement,  affixed  to  the  wood  by  studding  it  thick- 
ly over  with  broad-headed  nails  to  give  a  hold  to  the  cement, 
has  met  with  full  success ;  but  the  oxidation  of  the  metal, 
and  the  liability  to  accident  of  the  cement,  limit  their  effica- 
cy to  cases  where  they  can  be  renewed.  The  chemical  pro- 
cesses for  preserving  timber  from  decay  do  not  appear  to 
guard  them  in  salt  w^ater.  A  process,  however,  of  preserving 
timber  by  impregnating  it  with  coal  tar,  patented  in  this 
country  by  Professor  Renwick,  appears,  from  careful  experi- 
ments, also  to  be  efficacious  against  the  attack  of  the  ship- 
worm.  A  coating  of  Jeffery's  marine  glue,  when  impregnated 
with  some  of  the  insoluble  mineral  poisons  destructive  to 
animal  life,  is  said  to  subserve  the  same  end. 

242.  The  best  seasoned  timber  will  not  withstand  the  effects 
of  exposure  to  the  weather  for  a  much  greater  period  than 
twenty-five  years,  unless  it  is  protected  by  a  coating  of  paint 
or  pitch,  or  of  oil  laid  on  hot,  when  the  timber  is  partly 
charred  over  a  light  blaze.  These  substances  themselves,  be- 
ing of  a  perishable  nature,  require  to  be  renewed  from  time  to 
time,  and  will,  therefore,  be  serviceable  only  in  situations 
which  admit  of  their  renewal.  They  are,  moreover,  more 
hurtful  than  serviceable  to  unseasoned  timber,  as  by  closing 
the  pores  of  the  exterior  surface  they  prevent  the  moisture 
from  escaping  from  within,  and  therefore  promote  one  of  the 
chief  causes  of  decay. 

243.  Forest  Trees  of  the  United  States.  The  forests  of 
our  own  country  produce  a  great  variety  of  the  best  timber  for 


TIMBER. 


87 


every  purpose,  and  supply  abundantly  both  our  own  and  for- 
eiirn  markets.  The  followins;  o-enera  are  in  most  common 
use. 

244.  Oak.  About  forty-four  species  of  this  tree  are  enu- 
merated by  botanists,  as  found  in  our  forests  and  those  of 
Mexico.  The  most  of  them  afford  a  good  building  material, 
except  the  varieties  of  red  oak,  the  timber  of  which  is  weak 
and  decays  rapidly. 

The  White  Oak  {Quercus  Alba),  so  named  from  the  color 
of  its  bark,  is  amoug  the  most  valuable  of  the  species,  and  is 
in  very  general  use,  but  is  mostly  reserved  for  naval  construc- 
tions ;  its  trunk,  which  is  large,  serving  for  heavy  frame-work, 
and  the  roots  and  larger  bi'anches  affording  the  best  compass 
timber.  The  wood  is  strong  and  durable,  and  of  a  slightly 
reddish  tiuge ;  it  is  not  suitable  for  boards,  as  it  shrinks  "about 
-^Y  ill  seasoning,  and  is  very  subject  to  warp  and  crack. 

This  tree  is  found  most  abundantly  in  the  Middle  States. 
It  is  seldom  seen,  in  comparison  with  other  forest  trees,  in  the 
Eastern  and  Southern  States,  or  in  the  rich  valleys  of  the 
AYestern  States. 

Post  Oak  {Quercus  Ohtusiloha).  This  tree  seldom  attains 
a  greater  diameter  than  about  fifteen  inches,  and  on  this  ac- 
count is  mostly  used  for  posts,  from  w^hich  use  it  takes  its 
name.  The  wood  has  a  yellowish  hue,  and  close  grain  ;  is  said 
to  exceed  white  oak  in  strength  and  durability  ;  and  is  there- 
fore an  excellent  building  material  for  the  lighter  kinds  of 
frame- work.  This  tree  is  found  most  abundantly  in  the 
forests  of  Maryland  and  Virginia,  and  is  there  frequently 
called  Box  White  Oak,  and  Iron  Oak.  It  also  grows  in  the 
forests  of  the  Southern  and  Western  States,  but  is  rarely  seen 
farther  north  than  the  mouth  of  the  Hudson  River. 

Chestnut  White  Oak  [Quercus  Priims  Palustris).  The 
timber  of  this  tree  is  stroug  and  durable,  but  inferior  to  the 
two  preceding  species.  The  tree  is  abundant  from  North 
Carolina  to  Florida. 

Rock  Chestnut  Oak,  ( Quercus  Priiius  Monticolcc.)  The  tim- 
ber of  this  tree  is  mostly  in  use  for  naval  constructions,  for 
which  it  is  esteemed  inferior  oidy  to  tlie  white  oak.  The 
tree  is  found  in  the  Middle  States,  and  as  far  north  as  Ver- 
mont. 

Live  Oak  {Quercus  Virens).  The  wood  of  this  tree  is 
of  a  yellowish  tinge ;  it  is  heavy,  compact,  and  of  a  fine 
grain ;  it  is  stronger  and  more  durable  than  any  other  species, 
and  on  this  account  it  is  considered  invaluable  for  the  pur- 
poses of  ship-building,  for  which  it  is  exclusively  reserved. 


88 


CIVIL  ENGENEEKING. 


The  live  oak  is  not  found  farther  north  than  the  neighbor- 
hood of  Xorfolk,  Virginia,  nor  farther  inland  than  from  fif- 
teen to  twenty  miles  from  the  seacoast.  It  is  found  in  abun- 
dance along  the  coast  south,  and  in  the  adjacent  islands  as  far 
as  the  mouth  of  the  Mississi})])i. 

245.  Fine.  This  very  interesting  genus  is  considered  in- 
ferior only  to  the  oak,  from  the  excellent  timber  afforded  by 
nearly  all  of  its  species.  It  is  regarded  as  a  most  valuable 
building  material,  owing  to  its  strength  and  durability,  the 
straightness  of  its  fibre,  the  ease  with  which  it  is  wrought, 
and  its  applicability  to  all  the  purposes  of  constructions  in 
wood. 

Yellow  Pine  {Finns  Mitis).  The  heart-wood  of  this  tree 
is  fine-grained,  moderately  resinous,  strong  and  durable ;  but 
the  sap-wood  is  very  inferior,  decaying  rapidly  on  exposure  to 
the  weather.  The  timber  is  in  very  general  use  for  frame- 
work, &c. 

This  tree  is  found  throughout  our  country,  but  in  the  great- 
est abundance  in  the  Middle  States.  In  the  Southern  States 
it  is  known  as  Spruce  Fine  and  Short-leaved  Fine. 

Long-leaved  rine,  or  Southern  Pine  {Finns  Anstralis). 
This  tree  has  but  little  sap-wood,  and  the  resinous  matter  is 
uniformly  distributed  throughout  the  heart-wood,  which  pre- 
sents a  fine  compact  grain,  having  more  hardness,  strength, 
and  durability  than  any  other  species  of  the  pine,  owing  to 
which  cpialities  the  timber  is  in  very  great  demand. 

The  tree  is  first  met  wdth  near  Norfolk,  Virginia,  and  from 
this  point  south  it  is  abundantly  found. 

White  Pine,  or  Northern  Pine  {Finns  Strobus).  This  tree 
takes  its  name  from  the  color  of  its  wood,  w^hicli  is  white,  soft, 
light,  straight-grained,  and  durable.  It  is  inferior  in  strength 
to  the  species  just  described,  and  has,  moreover,  the  defect  of 
swelling  in  damp  weather.  Its  timber  is,  however,  in  great 
demand  as  a  good  building  material,  being  almost  the  only 
kind  in  use  in  the  Eastern  and  Northern  States  for  the  frame- 
work and  joinery  of  houses,  &c. 

The  finest  spe(iimens  of  this  tree  grow  in  the  forests  of 
Maine.  It  is  found  in  great  abundance  betw^een  the  -iSd  and 
47th  parallels,  N.  L. 

Among  the  forest  trees  in  less  general  use  than  the  oak 
and  pine,  the  Locust,  the  Chestnut,  X\\q  Fed  Cedar,  and  the 
Larch  hold  the  first  place  for  hardness,  strength,  and 
durability.  They  are  chiefly  used  for  the  frame-work  of  ves- 
sels. The  chestnut,  the  locust,  and  the  cedar  are  preferred  to 
all  other  trees^for  posts. 


METALS. 


89 


247.  The  Black  or  Double  Spruce  {Abies  Nigra)  also  af- 
fords an  excellent  material,  its  timber  being  strong,  durable, 
and  light. 

248.  The  Junvper  or  White  Cedar,  and  the  Cyjpress^  are 
very  celebrated  for  affording  a  material  which  is  very  light, 
and  of  great  durability  when  exposed  to  the  weather ;  owing 
to  these  qualities,  it  is  almost  exclusively  used  for  shingles 
and  other  exterior  coverings.  These  two  trees  are  found  in 
great  abundance  in  the  swamps  of  the  Southern  States. 


IX. 

METALS. 

The  metals  in  most  common  use  in  constructions  are  Iron, 
Cojjyjper,  Zinc,  Tin,  and  Lead. 

249.  IRON.  This  metal  is  very  extensively  used  for  the 
purposes  of  the  engineer  and  architect,  both  in  the  state  of 
Cast  Iron  and  Wrought  Iron. 

250.  Cast  Iron  is  one  of  the  most  valuable  building  materi- 
als, owing  to  its  great  strength,  hardness,  and  durability,  and 
the  ease  with  which  it  can  be  cast,  or  moulded,  into  the  best 
forms,  for  the  purposes  to  which  it  is  to  be  applied. 

251.  Cast  iron  is  divided  into  two  principal  varieties :  the 
Gray  cast  iron,  and  White  cast  iron.  There  exists  a  very 
marked  diiference  between  the  properties  of  these  two 
varieties.  There  are,  besides,  many  intermediate  varieties, 
which  partake  more  or  less  of  the  properties  of  these  two,  as 
they  approach,  in  their  external  appearances,  nearer  to  the  one 
or  the  other. 

252.  Gray  cast  iron,  when  of  a  good  quality,  is  slightly 
malleable  in  a  cold  state,  and  will  yield  readily  to  the  action 
of  the  file,  when  the  hard  outside  coating  is  removed.  This 
variety  is  also  sometimes  termed  soft  gray  cast  iron ;  it  is 
softer  and  tougher  than  the  white  iron.  W  hen  broken,  the  sur- 
face of  the  fracture  presents  a  graimlar  structure ;  the  color 
is  gray ;  and  the  lustre  is  what  is  termed  metallic,  resembling 
small  brilliant  particles  of  lead  strewed  over  the  surface. 

253.  White  cast  iron  is  very  hard  and  brittle;  when  re- 
cently broken,  the  surface  of  the  fracture  presents  a  distinctly-^ 


90 


CIVIL  ENGENEEKING. 


marked  cr3'stalline  structure ;  the  color  is  white ;  and  lustre 
vitreous,  or  bearing  a  resemblance  to  the  reflected  light  from 
an  aggregation  of  small  crystals. 

254.  itr.  Mallet,  in  a  very  able  Eeport  made  to  the  British 
Association  for  the  Advancement  of  Science,  remarking  on 
the  great  want  of  uniformity,  among  manufacturers  of  iron,  in 
the  terms  used  to  describe  its  different  varieties,  proposes  the 
following  nomenclature,  as  comprising  every  variety,  with 
their  distinctive  characters. 

Silvery.  Least  fusible ;  thickens  rapidly  when  fluid  by  a 
spontaneous  puddling ;  crystals  vesicular,  often  crystalline ; 
incapable  of  being  cut  by  chisel  or  file ;  ultimate  cohesion  a 
maximum  ;  elastic  range  a  minimum. 

Micaceous.  Yery  soft ;  greasy  feel ;  peculiar  micaceous 
appearance,  generally  owing  to  excess  of  manganese ;  soils 
the  fingers  strongly ;  crystals  large  ;  runs  very  fluid  ;  con- 
traction large. 

Mottled.  Tough  and  hard  ;  filed  or  cut  with  difficulty ; 
crystals  large  and  small  mixed  ;  sometimes  runs  thick ;  con- 
traction in  cooling  a  maximum. 

Bright  Gray.  Toughness  and  hardness  most  suitable  for 
working ;  ultimate  cohesion  and  elastic  range  generally  are 
balanced  most  advantageously;  crystals  uniform,  very  mi- 
nute. 

Dull  Gray.  Less  tough  than  the  preceding;  other  char- 
acters alike  ;  contraction  in  cooling  a  minimum. 

Dark  Gray.  Most  fusible ;  remains  long  fluid ;  exudes 
graphite  in  cooling ;  soils  the  fingers ;  crystals  large  and 
lamella ;  ultimate  cohesion  a  minimum,  and  elastic  range  a 
maxinnim. 

255.  The  gray  iron  is  most  suitable  where  strength  is  re- 
quired ;  and  the  white,  where  hardness  is  the  principal  re- 
quisite. 

256.  The  color  and  lustre,  presented  by  the  surface  of  a  re- 
cent fracture,  are  the  best  indications  of  the  quality  of  iron. 
A  uniform  dark  gray  color,  and  high  metallic  lustre,  are  in- 
dications of  the  best  and  strongest.  AYith  the  same  color,  but 
less  lustre,  the  iron  will  be  found  to  be  softer  and  weaker,  and 
to  crumble  readily.  Iron  without  lustre,  of  a  dark  and  mot- 
tled color,  is  the  sofest  and  weakest  of  the  gray  varieties. 

Iron  of  a  light  gray  color  and  high  metallic  lustre  is  usual- 
ly very  hard  and  tenacious.  As  the  color  approaches  to 
white,  and  the  metallic  lustre  changes  to  vitreous,  hai:dness 
and  brittleness  become  more  marked,  until  the  extremes  of  a 
dull,  or  grayish  white  color,  and  a  very  high  vitreous  lustre, 


METALS. 


91 


are  attained,  which  are  the  indications  of  the  hardest  and 
most  brittle  of  the  white  variety. 

257.  The  quality  of  cast  iron  may  also  be  tested,  by  strik- 
ing a  smart  stroke  with  a  hammer  on  the  edge  of  a  casting. 
If  the  blow  produces  a  slight  indentation,  without  any  appear- 
ance of  fracture,  it  shows  that  the  iron  is  slightly  malleable, 
and,  therefore,  of  a  good  quality  ;  if,  on  the  contrary,  the 
edge  is  broken,  it  indicates  brittleness  in  the  material,  and  a 
consequent  want  of  strength. 

258.  The  strength  of  cast  iron  varies  with  its  density ;  and 
this  element  depends  upon  the  temperature  of  the  metal  when 
drawn  from  the  furnace ;  the  rate  of  cooling ;  the  head  of 
metal  under  which  the  casting  is  made  ;  and  the  bulk  of  the 
casting. 

259.  The  density  of  iron  cast  in  vertical  moulds  increases, 
according  to  Mr.  Mallet's  experiments,  very  rapidly  from  the 
top  downward,  to  a  depth  of  about  four  feet  below  the  top  ; 
from  this  point  to  the  bottom,  the  rate  of  increase  is  very 
nearly  uniform.  All  other  circumstances  remaining  the 
same,  the  density  decreases  with  the  bulk  of  the  casting; 
hence  large  are  proportionally  weaker  than  small  castings. 

260.  From  all  of  these  causes,  by  wdiich  the  strength  of 
iron  may  be  influenced,  it  is  very  difficult  to  judge  of  the 
quality  of  a  casting  by  its  external  characters  ;  in  general, 
however,  if  the  exterior  presents  a  uniform  appearance,  de- 
void of  marked  inequalities  of  surface,  it  will  be  an  indica- 
tion of  uniform  strength. 

261.  The  economy  in  the  manufacture  of  cast  iron,  arising 
from  the  use  of  the  hot  blast,  has  naturally  directed  attention 
to  the  comparative  merits  between,  iron  produced  by  this  pro- 
cess and  that  from  the  cold  blast.  This  subject  has  been 
ably  investigated  by  Messrs.  Fairbairn  and  Hodgkinson,  and 
their  results  published  in  the  Seventh  Beport  of  the  British 
Association. 

Mr.  Hodgkinson  remarks  on  this  subject,  in  reference  to  the 
results  of  his  experiments :  "  It  is  rendered  exceedingly 
probable  that  the  introduction  of  a  heated  blast  into  the 
manufacture  of  cast  iron,  has  injured  the  softer  irons,  while 
it  has  frequently  mollified  and  improved  those  of  a  harder 
nature ;  and  considering  the  small  deterioration  that "  some 
"  irons  have  sustained,  and  the  apparent  benefit  to  those  of  " 
others,  "  together  with  the  great  saving  effected  by  the  heated 
blast,  there  seems  good  reason  for  the  process  becoming  as 
general  as  it  has  done." 

262.  From  a  number  of  specific  gravities  given  in  these 


92 


CIVIL  ENGINEERING. 


Reports,  the  mean  specific  gravity  of  cold  blast  iron  is  nearly 
7.091,  that  of  hot  blast,  7.021. 

263.  Mr.  Fairbairn  concludes  his  Report  with  these  obser- 
vations, as  the  results  of  the  investigations  of  himself  and 
Mr.  Hodgkinson  :  "  The  ultimatum  of  our  inquiries,  made  in 
this  way,  stands,  therefore,  in  the  ratio  of  strength,  1000  for 
the  cold  blast,  to  1024.8  for  the  hot  blast ;  leaving  the  small 
fractional  difference  of  24.8  in  favor  of  the  hot  blast." 

"  The  relative  powers  to  sustain  impact,  are  likewise  in 
favor  of  the  hot  blast,  being  in  the  ratio  of  1000  to  1226.3." 

264.  Wrought  Iron.  The  color,  lustre,  and  texture  of  a 
recent  fracture,  present,  also,  the  most  certain  indications  of 
the  quality  of  wrought  iron.  The  fracture  submitted  to  ex- 
amination, should  be  of  bars  at  least  one  inch  square  ;  or,  if 
of  flat  bars,  they  should  be  at  least  half  an  inch  thick  ;  other- 
wise, the  texture  will  be  so  greatly  changed,  arising  from  the 
greater  elongation  of  the  fibres,  in  bars  of  smaller  dimensions, 
as  to  present  none  of  those  distinctive  differences  observable 
in  the  fracture  of  large  bars. 

265.  The  surface  of  a  recent  fracture  of  good  iron,  presents 
a  clear  gray  color,  and  high  metallic  lustre ;  the  texture  is 
granular,  and  the  grains  have  an  elongated  shape,  and  are 
pointed  and  slightly  crooked  at  their  ends,  giving  the  idea  of 
a  powerful  force  having  been  employed  to  produce  the  frac- 
ture. When  a  bar,  presenting  these  appearances,  is  ham- 
mered, or  drawn  out  into  small  bars,  the  surface  of  fracture 
of  these  bars  will  have  a  very  marked  fibrous  appearance,  the 
filaments  being  of  a  white  color  and  very  elongated. 

266.  When  the  texture  is  either  laminated,  or  crystalline,  it 
is  an  indication  of  some  defect  in  the  metal,  arising  either 
from  the  mixture  of  foreign  ingredients,  or  else  from  some 
neglect  in  the  process  of  forging. 

267.  Burnt  iron  is  of  a  clear  gray  color,  with  a  slight 
shade  of  blue,  and  of  a  slaty  texture.    It  is  soft  and  brittle. 

268.  Cold  short  iron,  or  iron  that  cannot  be  hammered 
when  cold  without  breaking,  presents  nearly  the  same  appear- 
ance as  burnt  iron,  but  its  color  inclines  to  white.  It  is  very 
hard  and  brittle. 

269.  Hot  short  iron,  or  that  which  breaks  under  the  ham- 
mer when  heated,  is  of  a  dark  color  without  lustre.  This  de- 
fect is  usually  indicated  in  the  bar  by  numerous  cracks  on  the 
edges. 

270.  The  fibrous  texture,  which  is  developed  only  in  small 
bars  by  hammering,  is  an  inherent  quality  of  good  iron  ; 
those  varieties  which  are  not  susceptible  of  receiving  this  pe- 


METALS. 


93 


culiar  texture,  are  of  an  inferior  quality,  and  should  never  be 
used  for  purposes  requiring  great  strength  :  the  .filaments  of 
bad  varieties  are  short,  and  the  fracture  is  of  a  deep  color,  be- 
tween lead  gray  and  dark  gray. 

271.  The  best  wrought  iron  presents  two  varieties ;  the 
Hard  and  Soft.  The  hard  variety  is  very  strong  and  ductile. 
It  preserves  its  granular  texture  a  long  time  under  the  action 
of  the  hammer,  and  only  develops  the  fibrous  texture  when 
beaten,  or  drawn  out  into  small  rods  :  its  filaments  then  pre- 

♦  sent  a  silver- white  appearance. 

272.  The  soft  variety  is  weaker  than  the  hard ;  it  yields 
easily  to  the  hammer ;  and  it  commences  to  exhibit,  under  its 
action,  the  fibrous  texture  in  tolerably  large  bars.  The  color 
of  the  fibres  is  between  a  silver  white  and  light  gray. 

273.  Iron  may  be  naturally  of  a  good  quality,  and  still, 
fi'om  being  badly  refined,  not  present  the  appearances  which 
are  regarded  as  sure  indications  of  its  excellence.  Among 
the  defects  arising  from  this  cause  are  hlisters,  flaws^  and 
cinder-lioles.  Generally,  however,  if  the  surface  of  fracture 
presents  a  texture  partly  crystalline  and  partly  fibrous,  or  a 
nne  granular  texture,  in  which  some  of  the  grains  seem 
pointed  and  crooked  at  the  points,  together  with  a  light  gray 
color  without  lustre,  it  will  indicate  natural  good  qualities, 
which  require  only  careful  refining  to  be  fully  developed. 

274.  The  strength  of  wrought  iron  is  very  variable,  as  it 
depends  not  only  on  the  natural  qualities  of  the  metal,  but 
also  upon  the  care  bestowed  in  forging,  and  the  greater  or  less 
compression  of  its  fibres,  when  drawn  or  hammered  into  bars 
of  different  sizes. 

275.  In  the  Report  made  b}^  the  sub-committee,  Messrs. 
Johnson  and  Reeves,  on  the  strength  of  Boiler  Iron  {Journal 
of  Franklin  Institute^  vol.  20,  New  Series),  it  is  stated  that 
the  following  order  of  superiority  obtains  among  the  different 
kinds  of  pig  metal,  with  respect  to  the  malleable  iron  which 
they  furnish : — 1  Lively  gray  ;  2  White  ;  3  Mottled  gray  ; 
4  Dead  gray  ;  5  Mixed  metals. 

The  Report  states,  "  So  far  as  these  experiments  may  be 
considered  decisive  of  the  question,  they  favor  the  lighter 
complexion  of  the  cast  metal,  in  preference  to  the  darker  and 
mottled  varieties ;  and  they  place  the  mixture  of  different 
sorts  among  the  worst  modifications  of  the  material  to  be 
used,  where  the  object  is  mere  tenacity." 

276.  These  experiments  also  show  that  piling  iron  of  dif- 
ferent degrees  of  fineness  in  the  same  plate  is  injurious  to  its 
quality,  owing  to  the  consequent  inequality  of  the  welding. 


94 


CIVIL  ENGINEEEINQ. 


277.  From  these  experiments,  the  mean  specific  gravity  of 
boiler  iron  is  T.Y344,  and  of  bar  iron,  7.7254. 

278.  Durability  of  Iron.  Tlie  durability  of  iron,  under 
the  different  circumstances  of  exposure  to  which  it  may  be 
submitted,  depends  on  the  manner  in  which  the  casting  may 
have  been  made  ;  the  bulk  of  the  piece  employed  ;  the  more 
or  less  homogeneousness  of  the  mass ;  its  density  and  hard- 
ness. 

279.  Among  the  most  recent  and  able  researches  upon  the 
action  of  the  ordinary  corrosive  agents  on  iron,  and  the  pre- 
servative means  to  be  employed  against  them,  tliose  of  Mr. 
Mallet,  given  in  the  Report  already  mentioned,  hold  the  first 
rank.  A  brief  recapitulation  of  the  most  prominent  conclu- 
sions at  which  he  has  arrived,  is  all  that  can  be  attempted  in 
this  place. 

280.  When  iron  is  only  partly  immersed  in  water,  or 
wholly  immersed  in  water  composed  of  strata  of  different 
densities,  like  that  of  tidal  rivers,  a  voltaic  pile  of  one  solid 
and  two  fluid  bodies  is  formed,  which  causes  a  more  rapid 
corrosion  than  when  the  liquid  is  of  uniform  density. 

281.  The  corrosive  action  of  the  foul  sea  water  of  docks  and 
harbors  is  far  more  powerful  than  that  of  clear  sea  or  fresh 
water,  owing  to  the  action  of  the  hydro-sulphuric  acid  which, 
being  disengaged  from  the  mud,  impregnates  the  water,  and 
acts  on  the  iron. 

282.  In  clear  fresh  river  water,  the  corrosive  action  is  less 
than  under  any  other  circumstances  of  immersion  ;  owing  to 
the  absence  of  corrosive  agents,  and  the  firm  adherence  of  the 
oxide  formed,  which  j)resents  a  hard  coat  that  is  not  washed 
off  as  in  sea  water. 

283.  In  clear  sea  water,  the  rate  of  corrosion  of  iron  bars, 
one  inch  thick,  is  from  3  to  4  tenths  of  an  inch  for  cast  iron 
in  a  century,  and  about  6  tenths  of  an  inch  for  wrought 
iron. 

284.  Wrought  iron  corrodes  more  rapidly  in  hot  sea  water 
than  under  any  other  circumstances  of  immersion. 

285.  The  same  iron  when  chill  cast  corrodes  more  rapidly 
than  when  cast  in  green  sand ;  this  arises  from  the  chilled 
surface  being  less  uniform,  and  therefore  forming  voltaic 
couples  of  iron  of  different  densities,  by  wliich  the  rapidity 
of  corrosion  is  increased. 

286.  Castings  made  in  dry  sand  and  loam  are  more  durable 
under  water  than  those  made  in  green  sand. 

287.  Thin  bars  of  iron  corrode  more  rapidly  than  those  of 
more  bulk.    This  difference  in  the  rate  of  corrosion  is  more 


METALS. 


95 


striking  in  the  soft,  or  grapJiitic  specimens  of  cast  iron,  than 
in  the  hard  and  silvery.  It  is  caused  by  the  more  rapid  rate 
of  cooling  in  thin  than  in  "thick  l)ars,  by  which  the  density  of 
the  surface  of  the  former  becomes  less  uniform.  These 
causes  of  destructibility  may,  in  some  degree,  be  obviated  in 
castings  composed  of  ribbed  pieces,  by  making  the  ribs  of 
equal  thickness  with  the  main  pieces,  and  causing  them  to  be 
cooled  in  the  sand,  before  stripping  the  moulds. 

288.  The  hard  crust  of  cast  iron  promotes  its  durability ; 
when  this  is  removed  to  the  depth  of  one-fourth  of  an  inch, 
tlie  iron  corrodes  more  rapidly  in  botli  air  and  water. 

289.  Corrosion  takes  place  the  less  rapidly  in  any  variety 
of  iron,  in  proportion  as  it  is  more  homogeneous,  denser, 
harder,  and  closer  grained,  and  the  less  graphitic. 

290.  Preservatives  of  Iron. — The  more  ordinary  means 
nsed  to  protect  iron  against  the  action  of  corrosive  agents,  con- 
sist of  paints  and  varnishes.  These,  under  the  usual  circum- 
stances of  atmospheric  exposure  are  of  but  slight  efficacy,  and 
require  to  be  frequently  renewed.  In  water,  they  are  all 
rapidly  destroyed,  with  the  exception  of  boiled  coal-tar,  which 
when  laid  on  hot  iron,  leaves  a  bright  and  solid  varnish  of 
considerable  durability  and  protective  power. 

291.  The  rapidly  increasing  purposes  to  which  iron  has 
been  applied,  within  the  last  few  years,  has  led  to  researches 
upon  the  agency  of  electro -chemical  action,  as  a  means  of 
protecting  it  from  corrosion,  both  in  air  and  water.  Among 
the  processes  resorted  to  for  this  purpose,  that  of  zinhing^  or 
as  it  is  more  commonly  known,  galvanizing  iron  has  been 
most  generally  introduced.  The  experiments  of  Mr.  Mallet, 
on  this  process,  are  decidedly  against  zinc  as  a  permanent 
electro-chemical  protector.  Mr.  Mallet  states,  as  the  result  of 
his  observations,  that  zinc  applied  in  fusion,  in  the  ordinary 
manner,  over  the  whole  surface  of  iron,  will  not  preserve  it 
longer  than  about  two  years ;  and  that,  so  soon  as  oxidation 
commences  at  any  point  of  the  iron,  the  protective  power  of 
the  zinc  becomes  considerably  diminished,  or  even  entirely 
null.  Mr.  Mallet  concludes :  "  On  the  whole,  it  may  be 
affirmed  that,  under  all  circumstances,  zinc  has  not  yet  been 
so  applied  to  iron,  as  to  rank  as  an  electro-chemical  protector 
towards  it  in  the  strict  sense;  hitherto  it  has  not  become 
a  preventive,  but  merely  a  more  or  less  effective  palliative  to 
destruction." 

292.  In  extending  his  researches  on  this  subject,  with 
alloys  of  copper  and  zinc,  and  copper  and  tin,  Mr.  Mallet 
found  that  the  alloys  of  the  last  metals  accelerate  the  corro- 


96 


CIVIL  ENGINEERING. 


sion  of  iron,  when  voltaically  associated  with  it  in  sea  water ; 
and  that  an  alloy  of  the  two  first,  represented  by 
8Cu,  in  contact  with  iron,  protects  it  as  fully  as  zinc  alone, 
and  suffers  but  little  loss  from  the  electro-chemical  action  ; 
thus  presenting  a  2)rotective  energy  more  permanent  and  in- 
variable than  that  of  zinc,  and  giving  a  nearer  approxima- 
tion to  the  solution  of  the  problem,  "  to  obtain  a  mode  of 
electro-chemical  protection  such,  that  while  the  iron  shall  be 
preserved  the  protector  shall  not  be  acted  on,  and  whose  pro- 
tection shall  be  invariable." 

293.  In  the  course  of  his  experiments,  Mr.  Mallet  ascer- 
tained that  the  softest  gray  cast  iron  bears  such  a  voltaic 
relation  to  hard  bright  cast  iron,  when  immersed  in  sea  water 
and  voltaically  associated  with  it,  that  although  oxidation 
will  not  be  prevented  on  either,  it  will  still  be  greatly  retard- 
ed on  the  hard,  at  the  expense  of  the  soft  iron. 

294.  In  concluding  the  details  of  his  important  researches 
on  this  subject,  Mr.  Mallet  makes  the  following  judicious 
remarks :  "  The  engineer  of  observant  habit  will  soon  have 
perceived,  that  in  exposed  works  in  iron,  equality  of  section 
or  scantling,  in  all  parts  sustaining  equal  strain,  is  far  from 
insuring  equal  passive  power  of  permanent  resistance,  unless, 
in  addition  to  a  general  allowance  for  loss  of  substance  by 
corrosion,  this  latter  element  be  so  pro^dded  for,  that  it  shall 
be  equally  balanced  over  the  whole  structure  ;  or,  if  not, 
shall  be  compelled  to  confine  itself  to  portions  of  the  general 
structure,  which  may  lose  substance  with  injuring  its  sta- 
bility." 

"The  principles  we  have  already  established  sufiicieiltly 
guide  us  in  the  modes  of  effecting  this ;  regard  must  not  only 
be  had  to  the  contact  of  dissimilar  metals,  or  of  the  same  in 
dissimilar  fluids,  but  to  the  scantling  of  the  casting  and  of  its 
parts,  and  to  the  contact  of  cast  iron  with  wrought  iron  or 
steel,  or  of  one  sort  of  cast  iron  with  another.  Thus,  in  a  sus- 
pension bridge,  if  the  links  of  the  chains  be  hammered,  and 
the  pins  rolled,  the  latter,  wdiere  equally  exposed,  will  be  eat- 
en away  long  before  the  former.  In  marine  steam-boilers,  the 
rivets  are  hardened  by  hammering  until  cold  ;  the  plates, 
therefore,  are  corroded  through  round  the  rivets  before  these 
have  suffered  sensibly  ;  and  in  the  air-pumps  and  condensers 
of  engines  working  with  sea  water,  or  in  pit  work,  and  pumps 
lifting  mineralized  or  '  bad '  water  from  mines,  the  cast  iron 
perishes  first  round  the  holes  through  which  wrought  iron 
bolts,  &c.,  are  inserted.  And  abundant  other  instances  might 
be  given,  showing  that  the  effects  here  spoken  of  are  in  prac- 


METALS. 


97 


tical  operation  to  an  extent  that  should  press  the  means  of 
counteracting  them  on  the  attention  of  the  engineer." 

295.  Since  Mr.  Mallet's  Report  to  the  British  Association, 
he  has  invented  two  processes  for  the  protection  of  iron  from 
the  action  of  the  atmosphere  and  of  water  ;  the  one  by  means 
of  a  coating  formed  of  a  triple  alloy  of  zinc,  mercury,  and 
sodium,  or  potassium  ;  the  other  by  an  amalgam  of  palladium 
and  mercury.  ' 

296. .  The  first  process  consists  of  forming  an  alloy  of  the 
metals  used,  in  the  following  manner.  To  1,292  parts  of  zinc 
by  weight,  in  a  state  of  fusion,  202  parts  of  mercury  are  add- 
ed, and  the  metals  are  well  mixed,  by  stirring  with  a  rod  of 
dry  wood,  or  one  of  iron  coated  with  clay  ;  sodium,  or  potas- 
sium is  next  added,  in  small  quantities  at  a  time,  in  the  pro- 
portion of  one  pound  to  every  ton  by  weight  of  the  other  two 
metals.  The  iron  to  be  coated  w4th  this  alloy  is  first  cleared 
of  all  adhering  oxide,  by  immersing  it  in  a  warm  dilute  so- 
lution x)f  sulphuric,  or  of  hydrochloric  acid,  washing  it  in 
clear  cold  water,  and  detaching  all  scales,  by  striking  it  with 
a  hammer  ;  it  is  then  scoured  clean  by  the  hand  with  sand,  or 
emery,  under  a  small  stream  of  water,  until  a  bright  metallic 
lustre  is  obtained ;  while  still  wet,  it  is  immersed  in  a  bath 
formed  of  equal  parts  of  the  cold  saturated  solutions  of  chlo- 
ride of  zinc  and  sal-ammoniac,  to  which  as  much  more  solid  sal 
ammoniac  is  added  as  the  solution  will  take  up.  The  iron  is 
allowed  to  remain  in  this  bath  until  it  is  covered  by  minute 
bubbles  of  gas  ;  it  is  then  taken  out,  allowed  to  drain  a  few 
seconds,  and  plunged  into  the  fused  alloy,  from  which  it  is 
withdrawn  so  soon  as  it  has  acquired  the  same  temperature. 
When  taken  from  the  metallic  bath,  the  iron  should  be  plung- 
ed in  cold  water  and  well  washed. 

297.  Care  must  be  taken  that  the  iron  be  not  kept  too  long 
in  the  metallic  bath,  otherwise  it  may  be  fused,  owing  to  the 
great  aftinity  of  the  alloy  for  iron.  At  the  proper  fusing 
temperature  of  the  alloy,  about  680°  Fahr.,  it  will  dissolve 
plates  of  iron  one-eighth  of  an  inch  thick  in  a  few  seconds ; 
on  this  account,  whenever  small  articles  of  iron  have  to  be  pro- 
tected, the  affinity  of  the  alloy  for  iron  should  be  satisfied,  by 
fusing  some  iron  in  it  before  immersing  that  to  be  coated. 

298.  The  other  process,  which  has  been  termed  jpalladiumiz- 
ing,  consists  in  coating  the  iron,  prepared  as  in  the  first  pro- 
cess for  the  reception  of  the  metallic  coat,  with  *an  amalgam 
of  palladium  and  mercury. 

299.  Corrugated  Iron. — This  term  is  applied  to  sheet  iron 
prepared  by  being  moulded,  and  having  the  plane  surface 

7 


98 


CIVIL  ENGINEERING. 


broken  by  longitudinal  or  sectional  ridges,  for  the  purpose  of 
giving  the  sheet  great  stiffness  and  strength.  Corrugated  iron 
is  used  principally  for  roofing,  and  sometimes  in  place  of  brick' 
for  forming  the  arches  between  the  iron  beams  in  fire-proof 
Btructures. 

300.  Steel. — The  name  steel  is  given  to  a  compound  of  iron 
and  carbon,  in  which  the  amount  of  iron  is  usually  not  less 
than  97  per  cent.  Where  the  amount  of  carbon  is  less  than 
.0065,  the  compound  is  termed  steely  iron  ^  when  more  than 
1.8  the  compound  is  cast  iron. 

Steel,  like  iron,  is  seldom  pure,  containing  other  substances, 
of  which  sulphur  and  silicon  are  the  most  common. 

The  different  kinds  of  steel  are  named  either  from  the 
modes  of  manufacture,  or  their  appearance,  or  from  some  con- 
stituent, or  from  some  inventor's  process.  Thus  we  have 
natural  steels  obtained  directly  from  the  ores  and  bearing 
mostly  local  names ;  blistered,  shear,  tilted  and  crucible  or 
cast  steel ;  Woolz  or  Damask  steel ;  Bessemer's  and  ^lartin's 
steel ;  tungstein,  chromium,  and  titanium  steel. 

These  varieties  are  obtained  by  various  processes.  Thus 
we  have  \hQ  jouddling process  by  which  the  varieties  of  natural 
steel  are  made  ;  the  cementative  process ;  the  Martin-Siemems 
process ;  the  Bessemer  process,  &c. 

The  average  specific  gravity  of  natural  steels  is  7.5  ;  of 
tilted  steel  7.9 ;  cast  steel  7.8 ;  Bessemer  steel  from  7.79  to 
7.87 ;  chromium  steel  from  7.81  to  7.85. 

The  chromium  steel  is  said  to  possess  the  greatest  tensile 
strength ;  and  among  those  more  abundantly  manufactured 
the  Bessemer  still  raiis  highest  in  this  respect. 

COPPER. 

301.  The  most  ordinary  and  useful  application  of  this 
metal  in  constructions,  is  that  of  sheet  copper,  which  is  used 
for  roof  coverings,  and  like  purposes.  Its  durability  under 
the  ordinary  changes  of  atmospliere  is  very  great.  Sheet  cop- 
per, wlien  quite  thin,  is  apt  to  be  defective,  from  cracks  ari- 
sing from  the  process  ot  drawing  it  out.  These  may  be 
remedied,  when  sheet  copper  is  to  be  used  for  a  water-tight 
sheathing,  by  tinning  the  sheets  on  one  side.  Sheets  prepared 
in  this  way  have  been  found  to  be  very  durable. 

The  alloys  of  copper  and  zinc,  known  under  the  name  of 
hrass^  and  those  of  copper  and  tin,  known  as  hronze,  gun-metal^ 
and  hell-metal,  are,  in  some  cases,  substituted  for  iron,  owing 


METALS. 


99 


to  their  superior  hardness  to  copper,  and  being  less  readily 
oxidized  than  iron. 

zmc. 

302.  This  metal  is  used  mostly  in  the  form  of  sheets ;  and 
for  water-tight  sheathings  it  has  nearly  displaced  every  other 
kind  of  sheet  metal.  The  pure  metallic  surface  of  zinc  soon 
becomes  covered  with  a  very  thin,  hard,  transparent  oxide, 
which  is  unchangeable  both  in  air  and  water,  and  preserves 
the  metal  beneath  it  from  farther  oxidation.  It  is  this  prop- 
erty of  the  oxide  of  zinc,  which  renders  this  metal  so  valua- 
ble for  sheathing  purposes ;  but  its  durability  is  dependent 
upon  its  not  being  brought  into  contact  with  iron  in  the  pres- 
ence of  moisture,  as  the  galvanic  action  which  would  then 
ensue,  would  soon  destroy  the  zinc.  On  the  same  account 
zinc  should  be  perfectly  free  from  the  presence  of  iron,  as  a 
very  small  quantity  of  the  oxide  of  this  last  metal,  when  con- 
tained in  zinc,  is  found  to  occasion  its  rapid  destruction. 

Besides  the  alloys  of  zinc  already  mentioned,  this  metal 
alloyed  with  copper  forms  one  of  the  most  useful  solders;  and 
its  alloy  with  lead  has  been  proposed  as  a  cramping  metal  for 
uniting  the  parts  of  iron  work  together,  or  iron  work  to  other 
materials,  in  the  place  of  lead,  which  is  usually  employed  for 
this  purpose,  but  which  accelerates  the  destruction  of  iron  in 
contact  with  it. 

TIN. 

303.  The  most  useful  application  of  tin  is  as  a  coating  for 
sheet  iron,  or  sheet  copper :  the  alloy  which  it  forms,  in  this 
way,  upon  the  surfaces  of  the  metals  in  question,  preserves 
them  for  some  time  from  oxidation.  Alloyed  with  lead  it 
forms  one  of  the  most  useful  solders. 

LEAD. 

304.  Lead  in  sheets  forms  a  very  good  and  durable  roof 
covering,  but  it  is  inferior  to  both  copper  and  zinc  in  tenacity 
and  durability ;  and  is  very  apt  to  tear  asunder  on  inclined 
surfaces,  particularly  if  covered  with  other  materials,  as  in 
the  case  of  the  capping  of  water-tight  arches. 


100 


CIVIL  ENGINEERING. 


X. 

PAINTS  AND  VAENI8HES. 

305.  Paints  are  mixtures  of  certain  fixed  and  volatile  oils, 
chiefly  those  of  linseed  and  tni-pentine,  with  several  of  the 
metallic  salts  and  oxides,  and  other  substances  which  are  used 
either  as  pigments,  or  to  give  what  is  termed  a  bod]/  to  the 
paint,  and  also  to  improve  its  drying  properties. 

306.  Paints  are  mainly  used  as  protective  agents  to  secure 
wood  and  metals  from  the  destructive  action  of  air  and  water. 
This  they  but  imperfectly  effect,  owing  to  the  unstable  nature 
of  the  oils  that  enter  into  their  composition,  which  are  not 
only  destroyed  by  the  very  agents  against  which  they  are  used 
as  protectors,  but  by  the  chemical  changes  which  result  from 
the  action  of  the  elements  of  the  oil  upon  the  metallic  salts 
and  oxides. 

307.  Paints  are  more  durable  in  air  than  in  water.  In  the 
latter  element,  whether  fresh  or  salt,  particularly  if  foul, 
paints  are  soon  destroyed  by  the  chemical  changes  which  take 
place,  both  from  the  action  of  the  water  upon  the  oils,  and 
that  of  the  hydrosulphuric  acid  contained  in  foul  water  upon 
the  metallic  salts  and  oxides. 

308.  However  carefully  made  or  applied,  paints  soon  be- 
come permeable  to  water,  owing  to  the  very  minute  pores 
which  arise  from  tlie  chemical  changes  in  their  constituents. 
These  changes  will  have  but  little  influence  upon  the  preser- 
vative action  of  paints  upon  wood  exposed  to  tlie  effects  of 
the  atmosphere,  provided  the  wood  be  well  seasoned  before 
the  paint  is  applied,  and  that  the  latter  be  renewed  at  suitable 
intervals  of  time.  On  metals  these  changes  have  a  very  im- 
portant bearing.  The  permeability  of  the  paint  to  moisture 
causes  the  surface  of  the  metal  under  it  to  rust,  and  this  cause 
of  destruction  is,  in  most  cases,  promoted  by  the  chemical 
changes  which  the  paint  undergoes. 

309.  Varnishes  are  solutions  of  various  resinous  substances 
in  solvents  which  possess  the  property  of  drying  rapidly. 
They  are  used  for  the  same  purposes  as  paints,  and  have  gen- 
erally the  same  defects. 

310.  The  following  are  some  of  the  more  usual  composi- 
tions of  paints  and  varnishes. 


PAINTS  AND  VARNISH.  101 

White  Paint  (for  exposed  wood). 

White  lead,  ground  in  oil   80 

Boiled  oil   9 

Raw  oil   9 

Spirits  turpentine   4 

The  white  lead  to  be  ground  in  the  oil,  and  the  spirits  of 
turpentine  added. 

Black  Paint 

Lamp-black   28 

Litharge   1 

Japan  varnish   1 

Linseed  oil,  boUed   73 

Spirits  turpentine  ,   1 

Lead  Color. 

White  lead,  ground  in  oil   75 

Lamp-black   1 

Boiled  linseed  oil   23 

Litharge   0.5 

Japan  varnish   0.5 

Spirits  turpentine   2.5 

Grap^  or  Stone  Color  (for  buildings). 

White  lead  ground  in  oil   78 

Boiled  oil   9.5 

Raw  oil   9.5 

Spirits  of  turpentine.   3 

Turkey  umber   0.5 

Lamp-black   0.25 

Lackers  for  Cast  Iron. 

1.  —  Black  lead,  pulverized   12 

Red  lead..  '   13 

Litharge   5 

Lamp-black   5 

Linseed  oil   66 

2.  — Anti-corrosion   40  lbs. 

Grant's  black,  ground  in  oil   4  " 

Red  lead,  as  a  dryer   8  " 

Linseed  oil   4  gals. 

Spirits  turpentine   1  pint. 

Copal  Varnish. 

Gum  copal  (in  clean  lumps)   26.5 

Boiled  linseed  oil   42.5 

Spirits  tui-pentine   31 

Japan,  Varnish. 

Litharge   4 

Boiled  oil  '   87 

Spirits  turpentine   2 

Red  lead   6 

Umber   1 

Gum  shellac   8 

Sugar  of  lead   2 

White  vitriol  *  *  1 


102 


CIVIL  ENGINEKRING. 


The  proportions  of  the  above  compositions  are  given  in  100 
parts,  by  weight,  with  the  exception  of  lacker  2. 

The  beautiful  black  polish  on  the  Berlin  castings  for  orna- 
mental purposes,  is  said  to  be  produced  by  laying  the  follow- 
ing composition  on  the  hot  iron,  and  then  baking  it. 

Bitumen  of  India   0.5 

Eesin   0.5 

Drying  oil   1.0 

Copal,  or  amber  varnish   1.0 

Enough  oil  of  turpentine  is  to  be  added  to  this  mixture  to 
make  it  spread. 

311.  From  experiments  made  by  Mr.  Mallet,  on  the  pre- 
servative j)roperties  of  paints  and  varnishes  for  iron  immersed 
in  water,  it  appears  that  caoutchouc  varnish  is  the  best  for 
iron  in  hot  water,  and  asphaltum  varnish  under  all  other 
circumstances ;  but  that  boiled  coal-tar,  laid  on  hot  iron, 
forms  a  superior  coating  to  either  of  the  foregoing. 

312.  Varnish  for  Zineked  Iron.  Mr.  Mallet  recommends 
the  following  compositions  for  a  paint,  termed  by  him  200/0- 
gous  paint,  and  a  varnish  to  be  used  to  preserve  zineked  iron 
both  from  corrosion  and  from  fouling  in  sea  water. 

To  50  lbs.  of  foreign  asphaltum,  melted  and  boiled  in  an 
iron  vessel  for  three  or  four  hours,  add  16  lbs.  of  red  lead 
and  litharge  ground  to  a  fine  powder,  in  equal  proportions, 
with  10  gals,  of  drying  linseed  oil,  and  bring  the  whole  to  a 
nearly  boiling  temperature.  Melt,  in  a  second  vessel,  8  lbs. 
of  gum-anime  ;  to  which  add  2  gals,  of  drying  linseed  oil  at 
a  boiling  heat,  with  12  lbs.  of  caoutchouc  partially  dissolved 
in  coal-tar  naphtha.  Pour  the  contents  of  the  second  vessel 
into  the  first,  and  boil  the  whole  gentl}^,  until  the  varnish, 
when  taken  up  between  two  spatulas,  is  found  to  be  tough 
and  ropy.  This  composition,  when  quite  cold,  is  to  be  thinned 
down  for  use  with  from  30  to  35  gals,  of  spirits  of  turpentine, 
or  of  coal  naphtha. 

313.  It  is  recommended  that  the  iron  should  be  heated  be- 
fore receiving  this  varnish,  and  that  it  should  be  applied  with 
a  spatula,  or  a  flexible  slip  of  horn,  instead  of  the  ordinary 
brush. 

When  dry  and  hard,  it  is  stated  that  this  varnish  is  not 
acted  upon  by  any  moderately  diluted  acid  or  alkali ;  and, 
by  long  immersion  in  water,  it  does  not  form  a  partially  sol- 
uble hydrate,  as  is  the  case  with  purely  resinous  varnishes 
and  oil  paints.  It  can  with  difficulty  be  removed  by  a  sharp- 
pointed  tool ;  and  is  so  elastic,  that  a  plate  of  iron  covered 


PAINTS  AND  VAKNISH. 


103 


with  it  may  be  bent  several  times  before  it  will  become  de- 
tached. 

314.  Zoofagous  Paint.  To  100  lbs.  of  a  mixture  of  dry- 
ing linseed  oil,  red  lead,  sulphate  of  barytes,  and  a  little 
spirits  of  turpentine,  add  20  lbs.  of  the  oxy chloride  of  copper, 
and  3  lbs.  of  yellow  soap  and  common  rosin,  in  equal  propor- 
tions, with  a  little  w^ater. 

When  zincked  iron  is  exposed  to  the  atmosphere  alone,  the 
varnish  is  a  sufficient  protection  for  it ;  but  when  it  is  im- 
mersed in  sea  water,  and  it  is  desirable,  as  in  iron  ships,  to 
prevent  it  from  fouling,  by  marine  plants  and  animals  attach- 
ing themselves  to  it,  the  paint  should  be  used,  on  account  of 
its  poisonous  qualities.  The  paint  is  applied  over  the  varnish, 
and  is  allowed  to  harden  three  or  four  days  before  immer- 
sion. 

315.  Methods  of  Preserving  Exposed  Surfaces  of  Stone. 

Paints  and  similar  means  of  preservation  from  the  action  of 
the  weather  have  been  used  on  the  exposed  surfaces  of  ma- 
sonry composed  of  materials  that  were  found  not  to  with- 
stand well  this  action ;  besides  these,  preparations  of  the  alka- 
line silicates,  know^n  as  soluble  glass,  have  of  late  been 
recommended  as  of  a  more  durable  character  for  this  purpose. 
These  solutions  are  applied  either  by  syringes  or  by  a  brush 
to  the  surface  of  the  stone,  it  having  been  previously  cleansed 
of  all  extraneous  matter.  Three  applications  on  three  succes- 
sive days  are  said  to  be  sufficient  to  harden  and  preserve  any 
stone. 

Another  mode  of  effecting  the  same  object  has  been  pro- 
posed, which  is  to  use  two  solutions  of  mineral  substances 
which,  successively  applied  to  the  surface  of  the  stone,  shall 
form  an  insoluble  chemical  compound.  One  method  propos- 
ed is  to  saturate  the  stone  at  the  surface  with  a  weak  solution 
of  silicate  of  potash  or  soda,  on  which  a  solution  of  chloride  of 
calcium  or  barium  is  applied.  From  this  an  insoluble  silicate 
of  lime  or  barium  will  be  formed  in  the  pores  of  the  stone, 
which  will  render  it  weather-proof. 

Like  processes  have  been  used  for  dyeing  or  coloring  stone 
for  certain  architectural  effects.  For  this  purpose  some  of 
the  soluble  sulphates  are  used  in  various  combinations,  accord- 
ing to  the  color  to  be  obtained. 


CHAPTER  II. 


316.  Whatever  may  be  the  physical  structure  of  materials, 
whether  fibrous  or  granular,  experiment  has  shown  that  they 
all  possess  certain  general  properties,  among  the  most  impor- 
tant of  which  to  the  engineer  are  those  of  contraction,  elon- 
gation, deflection,  torsion,  lateral  adJiesion,  and  shearing,  and 
the  resistance  which  these  offer  to  the  forces  by  which  they 
are  called  into  action. 

Experimental  Researches  on  the  Strength  of  Materials. 

I.  General  Deductions  from  Experbients.  II.  Strength 
OF  Stone.  III.  Strength  of  Mortars  and  Concretes. 
lY.  Strength  of  Timber.  Y.  Strength  of  Cast  Iron. 
YI.  Strength  of  Wrought  Iron.  YII.  Strength  of 
Steel.  YIIl.  Strength  of  Copper.  IX.  Strength  of 
OTHER  Materials.  X.  Linear  Contraction  and  Expan- 
sion OF  Metals  and  other  Materials  from  Temperature. 
XL  Adhesion  of  Iron  Spikes  to  Timber. 

summary. 

1. 

general  deductions  from  experiments. 

Physical  properties  of  solid  bodies  and  the  various  experiments  to  test  them 
(Arts.  316-326). 

11. 

STRENGTH    OF  STONE. 

Resistance  of  stone  to  crushing-  and  transverse  strains  (Arts.  327-333). 
Practical  deductions  (Art.  334). 

Expansion  of  stone  from  increase  of  temperature  (Art.  335). 

III. 

STRENGTH  OF  MORTARS  AND  CONCRETES. 
Stren^h  of  moi-tars  (Arts.  33C-340). 

Strength  and  other  properties  of  Portland  cement  (Art.  341). 
Strength  of  concrete  and  be  ton  (Art.  342). 


STRENGTH  OF  MATERIALS. 


105 


lY. 

STRENGTH  OF  TIMBER. 

Resistance  to  tensile  strain  (Art.  343). 
Resistance  to  compressive  strain  (Art.  344). 
Resistance  of  square  pillars  (Art.  345). 
Resistance  to  transverse  strains  (Art.  346). 
Resistance  to  detrusion  (Art.  347). 

Y. 

STRENGTH  OF  CAST  IRON. 

Resistance  to  tensile  strain  (Art.  348). 
Resistance  to  compressive  strain  (Art.  349-354). 
Resistance  to  transverse  strain  (Art.  355-361). 

.  Influence  of  form  upon  the  strength  of  cast-iron  beams  (Art.  362-364). 
Formulas  for  determining  the  ultimate  strength  of  cast-iron  beams  of  the 

above  form  (Art.  365). 
Effect  of  horizontal  impact  upon  cast-iron  bars  (Art.  366-367). 

YI. 

STRENGTH  OF  WROUGHT  IRON. 

Resistance  to  tensile  strain  (Art.  368). 
Resistance  to  compressive  strain  (Art.  369-372). 
Resistance  of  iron  wire  to  impact  (Art.  373.) 
Resistance  to  torsion  (Art.  374). 

YII. 

STRENGTH  OF  STEEL. 
Strength  and  other  properties  of  steel  (Art.  375). 

YIII. 

STRENGTH  OF  COPPER. 
Resistance  to  tensile  and  compressive  strains  (Art.  376-377). 

IX. 

STRENGTH   OF  OTHER  METALS. 
Strength  of  cast  tin,  cast  lead,  gun-metal,  and  brass  (Art.  878). 

X. 

LINEAR   CONTRACTION  AND   EXPANSION   OF  METALS  AND  OTHER 
MATERIALS  FROM  TEMPERATURE. 

XI. 

ADHESION  OF  IRON  SPIKES  TO  TIMBER. 


106 


CIVIL  ENGINEERING. 


317.  All  solid  bodies,  when  submitted  to  strains  by  whicli 
any  of  these  properties  are  developed,  have,  within  certain 
limits,  termed  the  limits  of  elasticity ,  the  property  of  wholly 
or  partially  resuming  their  original  state,  when  the  strain  is 
taken  off. 

318.  To  what  extent  bodies  possess  the  property  of  total  re- 
covery of  form,  when  relieved  from  a  strain,  is  still  a  matter 
of  doubt.  It  has  been  generally  assumed,  that  the  elasticity 
of  a  material  does  not  undergo  permanent  injury  by  any  strain 
less  than  about  one-third  of  that  which  would  entirely  destroy 
its  force  of  cohesion,  thereby  causing  rupture.  But  from  the 
more  recent  experiments  on  this  point  made  by  Mr.  Hodgkin- 
son  and  others  on  cast  iron,  it  appears  that  the  restoring  power 
of  this  materia]  is  destroyed  by  very  slight  strains ;  and  it  is 
rendered  probable  that  this  and  most  other  materials  receive 
a  permanent  change  of  form,  or  set^  under  any  strain,  how- 
ever small. 

319.  The  extension,  or  contraction  of  a  solid,  may  be  effect- 
ed either  by  a  force  acting  in  the  direction  in  which  the  con- 
traction or  elongation  takes  place,  or  by  one  acting  trans- 
versely, so  as  to  bend  the  body.  Experiments  have  been  made 
to  ascertain,  directly,  the  proportion  between  the  amount  of 
contraction  or  elongation,  and  the  forces  by  which  they  are 
produced.  From  these  experiments,  it  results,  that  the  con- 
tractions or  elongations  are,  within  certain  limits,  proportional 
to  the  forces,  but  that  an  equal  amount  of  contraction,  or  elon- 
gation is  not  produced  by  the  same  amount  of  force.  From 
the  experiments  of  Mr.  Ilodgkinson  and  M.  Duleau,  it  ap- 
pears that  in  cast  and  malleable  iron  the  contraction  or  elon- 
gation caused  by  the  same  amount  of  pressure  or  tension  is 
nearly  equal ;  while  in  timber,  according  to  Mr.  Ilodgkinson, 
the  amount  of  contraction  is  about  f om'-fifths  of  the  elonga- 
tion for  the  same  force. 

320.  When  a  solid  of  any  of  the  materials  used  in  construc- 
tions is  acted  upon  by  a  force  so  as  to  produce  deflection,  ex- 
periment has  shown  that  the  fibres  towards  the  concave  side 
of  the  bent  solid  are  contracted,  while  those  towards  the  con- 
vex side  are  elongated ;  and  that,  between  the  fibres  which 
are  contracted  and  those  which  are  elongated,  othere  are  found 
which  have  not  undergone  any  change  of  length.  The  part 
of  the  solid  occupied  by  these  last  fibres  has  received  the  name 
of  the  neutral  line  or  neutral  axis. 

321.  The  hypothesis  usually  adopted,  with  respect  to  the 
circumstances  attending  this  kind  of  strain,  is  that  the  con- 


STRENGTH  OF  MATEEIALS. 


107 


tractions  and  elongations  of  the  fibres  on  each  side  of  the  neu- 
tral axis  are  proportional  to  their  distances  from  this  line  ;  and 
that,  for  slight  deflections,  the  neutral  axis  passes  through  the 
centre  of  gravity  of  the  sectional  area.  From  experiments, 
however,  by  Mr.  Hodgkinson  and  Mr.  Barlow,  on  bars  having 
a  rectangular  cross-section,  it  appears  that  the  neutral  axis,  in 
forged  iron  and  cast  iroii,  lies  nearer  to  the  concave  than  to 
the  convex  surface  of  the  bent  solid,  and,  probably,  shifts  its 
position  when  the  degree  of  deflection  is  so  great  as  to  cause 
rupture.  In  timber,  according  to  Mr.  Barlow,  the  neutral 
axis  lies  nearest  to  the  convex  surface ;  and,  from  his  experi- 
ments on  solids  of  forged  iron  and  timber  with  a  rectangular 
sectional  figure,  he  places  the  neutral  axis  at  about  three- 
eighths  of  the  depth  of  the  section  from  the  convex  side  in 
timber,  and  between  one-third  and  one-fifth  of  the  depth  of 
the  section  from  the  concave  side  in  forged  iron. 

322.  When  the  strain  to  which  a  solid  is  subjected  is  suf- 
ficiently great  to  destroy  the  cohesion  between  its  particles 
and  cause  rupture,  experiment  has  shown  that  the  force  pro- 
ducing this  eifect,  whether  it  act  by  tension,  so  as  to  draw  the 
fibres  asunder,  or  by  compression,  to  crush  them,  is  propor- 
tional to  the  sectional  area  of  the  solid. 

323.  From  experiments  made  to  ascertain  the  circumstan- 
ces of  rupture  by  a  tensile  force,  it  appears  that  the  solid  torn 
apart  exhibits  a  surface  of  fracture  more  or  less  even,  accord- 
ing to  the  nature  of  the  material. 

324.  Most  of  the  experiments  on  the  resistance  to  rupture 
by  compression,  have  been  made  on  small  cubical  blocks,  and 
have  given  a  measure  of  this  resistance  greater  than  can  be 
depended  U23on  in  practical  applications,  when  the  height  of 
the  solid  exceeds  three  times  the  radius  of  its  base.  This 
point  has  been  very  fully  elucidated  in  the  experiments  of 
Mr.  Hodgkinson  upon  the  rupture  by  compression  .of  solids 
with  circular  and  rectangular  bases.  These  experiments  go 
to  prove  that  the  circumstances  of  rupture,  and  the  resistance 
offered  by  the  solid,  vary  in  a  constant  manner  with  its  height, 
the  base  remaining  the  same.  In  columns  of  cast  iron,  with 
circular  sectional  areas,  it  was  found  that  the  resistance  re- 
mained constant  for  a  height  less  than  three  times  the  radius 
of  the  base  ;  that,  from  this  height  to  one  equal  to  six  times 
the  radius  of  the  base,  the  resistance  still  remained  constant, 
but  was  less  than  in  the  former  case ;  and  that,  for  any  height 
greater  than  six  times  the  radius  of  the  base,  the  resistance 
decreased  with  the  height.  In  the  two  first  cases  the  solids 
were  found  to  yield  either  by  the  upper  portion  sliding  off 


108 


CIYIL  ENGrNEEKING. 


upon  the  lower,  in  the  direction  of  a  plane  making  a  constant 
angle  with  the  axis  of  the  solid ;  or  else  by  separating  into 
conical  or  wedge-shaped  blocks,  having  the  upper  and  lower 
surfaces  of  the  solid  as  their  bases,  the  angle  at  the  apex  be- 
ing double  that  made  by  the  plane  and  axis  of  the  solid. 
"With  regard  to  the  resistance,  it  was  found  that  they  varied  in 
the  ratio  of  the  area  of  the  bases  of  the  solids.  Where  the 
height  of  the  solid  was  greater  than  six  times  the  radius  of 
the  base,  rupture  generally  took  place  by  bending. 

325.  From  experiments  by  Mr.  Ilodgkinson,  on  wood  and 
other  substances,  it  would  appear  that  like  circumstances  ac- 
company the  rupture  of  all  materials  by  compression  ;  that  is, 
within  certain  limits,  they  all  yield  by  an  oblique  surface  of 
fracture,  the  angle  of  which  w^ith  the  axis  of  the  solid  is  con- 
stant for  the  same  material ;  and  that  the  resistance  offered 
within  these  limits  are  proportional  to  the  areas  of  the 
bases. 

326.  Among  the  most  interesting  deductions  drawn  by  Mr. 
Hodgkinson,  from  the  wide  range  of  his  experinients  upon  the 
strength  of  materials,  is  the  one  which  points  to  the  existence 
of  a  constant  relation  between  the  resistances  offered  by  ma- 
terials of  the  same  kind  to  rupture  from  compression,  tension, 
and  a  transverse  strain.  The  following  Table  gi\'es  these  re- 
lations, assuming  the  measure  of  the  crushing  force  at  1000. 


DESCEIPTION  OF  MATERIAL. 

Crushing  force  per 
square  inch. 

Mean  tensile  force 
per  square  inch. 

Mean  transverse  force 
of  a  bar  1  inch  square 
and  1  foot  long. 

Timber  

1000 

1900 

85.1 

1000 

158 

19.8 

1000 

100 

9.8 

Glass  (plate  and  crown). 

1000 

123 

10 

STRENGTH  OF  STONE. 


109 


II. 

STRENGTH  OF  STONE. 

327.  The  marked  difference  in  the  structure  and  in  the 
proportions  of  the  component  elements  frequently  observed  in 
stone  from  the  same  quarry  would  lead  to  the  conclusion 
that  corresponding  variations  would  be  found  in  the  strength 
of  stones  belonging  to  the  same  class,  a  conclusion  which  ex- 
periment has  confirmed.  The  experiments  made  by  different 
individuals  on  this  subject,  from  not  having  been  conducted 
in  the  same  manner,  and  from  the  omission  in  most  cases  of 
details  respecting  the  structure  and  component  elements  of 
the  material  tried,  have,  in  some  instances,  led  to  contradic- 
tory results.  A  few  facts,  however,  of  a  general  character, 
have  been  ascertained,  which  may  serve  as  guides  in  ordinary 
cases  ;  but  in  important  structures,  where  heavy  pressures  are 
to  be  sustained,  direct  experiment  is  the  only  safe  course  for 
the  engineer  to  follow,  in  selecting  a  material  from  untried 
quarries. 

328.  Owing  to  the  ease  with  which  stones  generally  break 
Tinder  a  percussive  force,  and  from  the  comparatively  slight 
resistance  they  offer  to  a  tensile  force  and  to  a  transverse 
strain,  they  are  seldom  submitted  in  structures  to  any  other 
strain  than  one  of  compression ;  and  cases  but  rarely  occur 
where  this  strain  is  not  greatly  beneath  that  which  the  better 
class  of  building  stones  can  sustain  permanently,  without  un- 
dergoing any  change  in  their  physical  properties.  Where  the 
durability  of  the  stone,  therefore,  is  well  ascertained,  it  may 
be  safely  used  without  a  resort  to  any  specific  experiment 
upon  its  strength,  whenever,  in  its  structure  and  general  ap- 
pearance, it  resembles  a  material  of  the  same  class  known  to 
be  good. 

329.  The  following  table  exhibits  the  principal  results  of 
experiments  made  by  Mr.  G.  Rennie,  and  published  in  the 
Philosophical  Transactions  of  1818.  The  stones  tried  were 
in  small  cubes,  measuring  one  and  a  half  inches  on  the  edge. 
The  table  gives  the  pressure,  in  tons,  borne  by  each  superficial 
inch  of  the  stone  at  the  moment  of  crushing. 


110 


CIVIL  ENGINEERING. 


DESCRIPTION  OF  STONE. 


Granites. 

Aberdeen,  (bltte)  

Peterhead  

Cornwall  

Sandstones. 

Dundee  

Do  

Derby  (red  and  friable)  

Limestones. 

Marble  (white-veined  Italian) . .  , . . 

Do.    {lohite  Brabant)  

Limerick  (black  compact)  , 

Devonshire  {red  marble)  

Portland  stone  (fine-grained  oolite) 


Crushing  w'ght. 


The  following  results  are  taken  from  a  series  of  experiments 
made  under  the  direction  of  Messrs.  Bramah  &  Sons,  and 
published  in  Vol.  1,  Transactions  of  the  Institution  of 
Civil  Engineers.  The  first  column  of  numbers  gives  the 
weights,  in  tons,  borne  by  each  superficial  inch  when  the 
Btones  commenced  to  fracture ;  the  second  column  gives  the 
crushing  weight,  in  tons,  on  the  same  surface. 


DESCEIPTION  OF  STONE. 

Aver,  weight  pro- 
ducing fractvires. 

Average  cnishing 
weight. 

Granites. 

4.77 

6.64 

4.13 

4.64 

3.94 

6.19 

3.52 

5.48 

2.88 

4.88 

2.86 

4.36 

Sandstones. 

2.87 

3.94 

1.89 

2.97 

1.09 

2.06 

1.00 

1.06 

The  following  table  is  taken  from  one  published  in  Vol, 


8TEENGTH  OF  STONE. 


Ill 


2,  Civil  Engineer  and  Architects  Joiirnal,  which  forms  a 
part  of  the  Report  on  the  subject  of  selecting  stone  for  the 
Kew  Houses  or  Parliament.  The  specimens  submitted  to  ex- 
periment were  cubical  blocks  measuring  two  inches  on  an 
edge. 


DESCRIPTION  OF  STONE. 

Specific  gravity. 

Weight  produ- 
cing fracture. 

Crushing  w'ght 

Sandstones. 

2. 232 

1.89 

3.5 

T^qtIatt"    Tin  1a 

2.628 

2.75 

3  1 

2.229 

0.82 

1*.75 

2.247 

1.51 

2.21 

2.338 

0.88 

1.64 

'M'fi.fmfijiiftTi,  T A.Tn  f,stonp,s 

2.816 

2.21 

3.75 

2.147 

1.03 

1.92 

2.134 

0.75 

1.73 

2.138 

0.32 

1.92 

Oolites. 

2.182 

0.75 

1.04 

1.839 

0.56 

0.66 

2.145 

0.95 

1.75 

2.045 

0.69 

1.18 

Limestones. 

2.090 

0.50 

0.79 

2.481 

1.32 

3.19 

2.260 

0.69 

1.80 

The  numbers  of  the  first  column  give  the  specific  gravities ; 
those  in  the  second  column  the  weight  in  tons  on  a  square 
inch,  when  the  stone  commenced  to  fracture ;  and  those  in  the 
third  the  crushing  weight  on  a  square  inch. 

The  following  table  exhibits  the  results  of  experiments  on 
the  resistance  of  stone  to  a  transverse  strain,  made  by  Colonel 
Pasley,  on  prisms  4  inches  long,  the  cross  section  being  a 
square  of  2  inches  on  a  side ;  the  distance  between  the  points 
of  support  3  inches. 

330.  The  conductors  of  the  experiments  on  the  stone  for 
the  New  Houses  of  Parliament,  Messrs.  Daniell  and  Wheat- 
stone,  who  also  made  a  chemical  analysis  of  the  stones,  and 
applied  to  them  Brard's  process  for  testing  their  resistance  to 


112 


CIVIL  ENGINEERING. 


DKSCRIPTION  OF  STONE. 


Weight  of  stone 
per  cubic  foot 
in  lbs. 


Average  breaking 
weight  in  lbs. 


1.  Kentish  Rag  

2.  Yorkshire  Landing, 

3.  Cornish  granite  

4.  Portland  

6.  Craigleith  

6.  Bath  

7.  Well-burned  bricks. 

8.  Inferior  bricks  


165.69 
147.67 
172.24 
148.08 
144.47 
122.58 
91.71 


4581 
2887 
2808 
2682 
1896 
666 
752 
329 


fi'ost,  have  appended  the  following  conclusions  from  their 
experiments : — "  If  the  stones  be  divided  into  classes,  accor- 
ding to  their  chemical  composition,  it  will  be  found  that  in 
all  stones  of  the  same  class  there  exists  generally  a  close  rela- 
tion between  their  various  phj'sical  qualities.  Thus  it  will  be 
observed  that  the  specimen  which  has  the  greatest  specific 
gravity  possesses  the  greatest  cohesive  strength,  absorbs  the 
least  quantity  of  water,  and  disintegrates  the  least  by  the  pro- 
cess which  imitates  the  effects  of  weather.  A  comparison  of 
all  the  experiments  shows  this  to  be  the  general  rule,  though 
it  is  liable  to  individual  exceptions." 

"  But  this  will  not  enable  us  to  compare  stones  of  different 
classes  together.  The  sandstones  absorb  the  least  quantity  of 
water,  but  they  disintegrate  more  than  the  magnesian  lime- 
stones, which,  considering  their  compactness,  absorb  a  great 
deal." 

331.  Like  conclusions  to  the  preceding  were  reached  by  a 
commission  for  testing  the  properties  or  some  of  the  stones 
and  marbles  used  in  the  construction  of  the  Capitol  at  "Wash- 
ington. 

But  few  experiments  have  been  made  upon  the  strength 
and  other  properties  of  the  building  stones  of  the  United 
States,  and  those  of  a  local  character.  From  the  reports  of  a 
public  commission,  and  of  Professor  R.  Johnson,  on  the  mar- 
bles and  micaceous  stratified  stones  used  in  the  walls  and 
foundations  of  the  Capitol  at  Washington,  the  same  general 
conclusions  were  arrived  at  as  in  the  report  of  Messrs.  Daniell 
and  Wheatstone  above  cited.  The  strength  of  the  marbles 
submitted  to  expei'iment  varied  from  about  seven  thousand 
to  twenty-four  thousand  pounds  to  the  square  inch  ;  the  mica- 
ceous stones  used  in  the  foundations  averaged  about  fifteen 
thousand  pounds  to  the  square  inch ;  some  specimens  of  sand- 
stone gave  about  five  thousand  pounds  to  the  square  inch ;  and 


STEENGTH  OF  STONE. 


113 


one  of  sienite  about  twenty-nine  thousand  pounds  to  the  square 
inch. — Bejport  of  the  Architect  of  Public  Buildings,  Dec.  1, 
1852. 

332.  Kondelet,  from  a  numerous  series  of  experiments  on 
the  same  subject,  published  in  his  work,  Art  de  Bdtir^  has 
arrived  at  like  conclusions  with  regard  to  the  relations  between 
tlie  specific  gravity  and  strength  of  stones  belonging  to  the 
same  class. 

Among  the  results  of  the  more  recent  experiments  on  this 
subject,  those  obtained  by  Mr.  Hodgkinson,  showing  the 
relation  between  the  crushing,  the  tensile,  and  the  transverse 
strength  of  stone,  have  already  been  given. 

M.  Vicat,  in  a  memoir  on  the  same  subject,  published  in 
the  Annates  des  Fonts  et  Ghaussees,  1833,  has  arrived  at  an 
opposite  conclusion  from  Mr.  Hodgkinson,  stating  as  the  re- 
sults of  his  experiments,  that  no  constant  relation  exists  be- 
tween the  crushing  and  tensile  strength  of  stone  in  geneial, 
and  that  there  is  no  other  means  of  determining  these  two 
forces  but  by  direct  experiment  in  each  case. 

333.  The  influence  of  form  on  the  strength  of  stone,  and 
the  circumstances  attending  the  rupture  of  hard  and  soft 
stones,  have  been  made  the  subject  of  particular  experiments 
by  Rondelet  and  Yicat.  Their  experiments  agree  in  estab- 
lishing the  points  that  the  crushing  weight  is  in  proportion  to 
the  area  of  the  base.  Yicat  states,  more  generally,  that  the 
permanent  weights  borne  by  similar  solids  of  stone,  under  like 
circumstances,  will  be  as  the  squares  of  their  homologous 
sides.  These  two  authors  agree  on  the  point  that  the  circular 
form  of  the  base  is  the  most  favorable  to  strength.  They 
differ  on  most  other  points,  and  particularly  on  the  manner  im 
which  the  different  kinds  of  stone  yield  by  rupture. 

334.  Practical  Deductions.  Were  stones  placed  under 
the  same  circumstances  in  structures  as  in  the  experiments 
made  to  ascertain  their  strength,  there  would  be  no  difficulty  in 
assigning  the  fractional  part  of  the  weight  termed  the  worh- 
ing  strain  or  working  load  which,  in  the  comparatively  short 
period  usually  given  to  an  experiment,  will  crush  them,  could, 
be  borne  by  them  permanently  with  safety.  But,  indepen- 
dently of  the  accidental  causes  of  destruction  to  which  struo 
tures  are  exj^osed,  imperfections  in  the  material  itself,  as  well 
as  careless  workmanship,  from  which  it  is  often  placed  in  the 
most  unfavorable  circumstances  of  resistance,  require  to  be 
guarded  against.  M.  Yicat,  in  the  memoir  before  mentioned, 
states  that  a  permanent  strain  of  yVtt  the  crushing  force  of 
experiment  may  be  borne  by  stone  without  danger  of  impair- 

8  * 


114 


CIVIL  ENGINEERING. 


ing  its  cohesive  strength,  provided  it  be  placed  under  the 
most  favorable  circumstances  of  resistance.  This  fraction  of 
the  crushing  weiglit  of  experiment  is  greater  than  ordinary- 
circumstances  would  justify,  and  it  is  recommended  in  prac- 
tice not  to  submit  any  stone  to  a  greater  permanent  strain 
than  one-tenth  of  the  crushing  weight  of  experiments  made 
on  small  cubes  measuring  about  two  inches  on  an  edge. 

Other  authorities  state  that  cut  stone  in  cases  like  the  vous- 
soirs  of  arches  and  stone  pillars  should  not  be  subjected  to  a 
working  strain  greater  than  ^^^th  of  the  crushing  weight  of 
experiment. 

The  following  table  show^s  the  permanent  strain,  and 
crushing  weight,  for  a  square  foot  of  the  stones  in  some  of 
the  most  remarkable  structures  in  Europe. 


Permanent 

Crushing 

strain. 

weight. 

33330 

536000 

39450 

537000 

60000 

456000 

90000 

900000 

Lower  courses  of  the  piers  of  the  Bridge  of  Neuilly. . 

3000 

570000 

The  stone  employed  in  all  the  structures  enumerated  in  the 
Table,  is  some  variety  of  limestone. 

335.  Expansion  of  Stone  from  Increase  of  Tempera- 
ture. Experiments  have  been  made  in  this  comitry  by  Prof. 
Bartlett,  and  in  England  by  Mr.  Adie,  to  ascertain  the  expan- 
sion of  stone  for  every  degree  of  Fahrenheit.  The  experi- 
ments of  Prof.  Bartlett  give  the  following  results : 

Granite  expands  for  every  degree  000004825 

Marble  :  000005668 

Sandstone  000009533 


STKENGTH  OF  MOETAES. 


115 


Tahle  of  the  Expansion  of  Stone^  etc.^from  the  Experiments 
of  Alexander  J.  Adie,  Civil  Engineer^  Edinburgh. 


DESCBEPTION  OF  STONE. 


Roman  cement  

Sicilian  white  marble  j 

Carrara  marble  -j 

Sand-stone  {Craigleith)  

Slate  {Welch)  

Red  granite  {Peterhead)..  -| 

Arbroath  pavement  

Caithness  pavement  

Green-stone  {Ratho)  

Gray  granite  {Aberdeen). . . 

Best  stock  brick  

Fire  brick  

Black  marble  (  Oalway)  


»,^\?^:,^i^>ecimal  of 
an  mch  on 

23    inches  ^"f^Ji 

for  180°  P.  ^ 


.6330043 

.0325392 

.0253946 

.0274344 

.0150405 

.0270093 

.02380.59 

.0220416 

.0206266 

.0206052 

.020.5788 

.0186043 

.01815695 

.0126542 

.0113334 

.0102394 


.0014349 

.0014147 

.00110411 

.0011928 

.0006539 

.0011743 

.0010376 

.0009583 

.0008968 

.0008985 

.0008947 


.00078943 
.0005502 
.0004928 
.00044519 


Decimal  of 
length  for 
1°  F. 


.00000750 
.00000780 
.00000613 
.00000662 
.00000363 
.00000652 
.00000576 
.00000632 
.00000498 
.00000499 
.00000497 
.00000449 
.00000438 
.00000306 
.00000274 
.00000247 


Remarks. 


(  One  experiment  {moist). 

I  Mean  of  three  {dry). 

J  One  experiment  {moist). 

\  Mean  of  two  {dry). 
Mean  of  four  experiments. 
Mean  of  three  do, 
J  One  experiment  {moist), 

I  Mean  of  two  {dry). 
Mean  of  four  experiments. 
Mean  of  three  do. 
Mean  of  three  do. 
Mean  of  two  do. 
Mean  of  two  do. 
Mean  of  two  do. 
Mean  of  three  do. 


III. 

STEENGTH  OF  MOKTAKS  AND  CONCRETES. 


336.  Strength  of  Mortars.  A  very  wide  range  of  experi- 
ments has  been  made,  within  a  few  years  back,  by  engineers 
both  at  home  and  abroad,  upon  the  resistance  offered  by  mor- 
tars to  a  transversal  strain,  with  a  view  to  compare  their  qual- 
ities, both  as  regards  their  constituent  elements  and  the 
processes  followed  in  their  manipulation.  As  might  naturally 
have  been  anticipated,  these  experiments  have  presented  very 
diversified,  and  in  many  instances,  contradictory  results.  The 
general  conclusions,  however,  drawn  from  them,  have  been 
nearly  the  same  in  the  majority  of  cases ;  and  they  furnish 
the  engineer  with  the  most  reliable  guides  in  this  important 
branch  of  his  art. 

337.  The  usual  method  of  conducting  these  experiments  has 
been  to  subject  small  rectangular  prisms  of  mortas,  resting  on 
points  of  support  at  their  extremities,  to  a  transversal  strain 
applied  at  the  centre  point  between  the  bearings.  This,  per- 
haps, is  as  unexceptionable  and  convenient  a  method  as  can 
be  followed  for  testing  the  comparative  strength  of  mortars. 


116 


CIVIL  ENGINEERING. 


338.  M.  Yicat,  in  the  work  already  cited,  gives  the  follow- 
ing as  the  average  resistances  on  the  square  inch  offered  by 
mortars  to  a  force  of  traction  ;  the  deductions  being  drawn 
from  experiments  on  llie  resistance  to  a  transversal  strain. 

Mortars  of  very  strong  hydraulic  lime. . 
"  ordinary  do.  ... 

"  medium  do.  . . . 

"  common  lime      /      do.  ... 

"  do.  (bad  quality) 

These  experiments  were  made  upon  prisms  a  year  old, 
which  had  been  exposed  to  the  ordinary  changes  of 
weather.  With  regard  to  the  best  hydraulic  mortars  of  the 
same  age  which  had  been,  during  that  same  period,  either 
immersed  in  water,  or  buried  in  a  damp  position,  M.  Yicat 
states  that  their  average  tenacity  may  be  estimated  at  140 
pounds  on  the  s(piare  inch. 

339.  General  Treussart,  in  his  work  on  hydraulic  and  com- 
mon mortars,  has  given  in  detail  a  large  number  of  experi- 
ments on  the  transversal  strength  of  artifical  hydraulic  mor- 
tars, made  by  submitting  small  rectangular  parallelopipeds 
of  mortar,  six  inches  in  length  and  two  inches  square,  to  a 
transversal  strain  applied  at  the  centre  point  between  the 
bearings,  w^hich  were  four  inches  apart.  From  these  experi- 
ments he  deduces  the  following  practical  conclusions. 

Tliat  when  the  parallelopipeds  sustain  a  transversal  strain 
varying  between  220  and  330  pounds,  the  corresponding  mor- 
tar will  be  suitable  for  common  gross  masonry  ;  but  that  for 
important  hydraulic  works  the  parallelopipeds  should  sustain, 
before  yielding,  from  330  to  440  pounds. 

340.  The  only  published  experiments  on  this  subject  made 
in  tliis  country  are  those  of  Colonel  Totten,  appended  to  his 
translation  of  General  Treussart's  work.  The  results  of  these 
experiments  are  of  peculiar  value  to  the  American  engineer, 
as  they  were  made  upon  materials  in  very  general  use  on  the 
public  Avorks  throughout  the  country. 

From  these  experiments  Colonel  Totten  deduces  the  follow- 
ing general  results : 

1st.  That  mortar  of  hydraulic  cement  and  sand  is  the  strong- 
er and  harder  as  the  quality  of  sand  is  less. 

2d.  That  common  mortar  is  the  stronger  and  harder  as  the 
quantity  of  sand  is  less. 

3d.  That  any  addition  of  common  lime  to  a  mortar  of 
hydraulic  cement  and  sand  weakens  the  mortar,  but  that  a 
little  lime  may  be  added  without  any  considerable  diminution 
of  the  strength  of  the  mortar,  and  with  a  saving  of  expense. 


170  pounds. 
.140  " 
.100  " 
.  40  " 
.  10  " 


STRENGTH  OF  MOKTAKS. 


117 


4th.  The  strength  of  common  mortars  is  considerably 
improved  by  the  addition  of  an  artificial  puzzolana,  but  more 
so  by  the  addition  of  an  hydraulic  cement. 

6th.  Fine  sand  generally  gives  a  stronger  mortar  than 
coarse  sand. 

6th.  Lime  slaked  by  sprinkling  gave  better  results  than 
lime  slaked  by  drowning.  A  few  experiments  made  on  air- 
slaked  lime  were  unfavorable  to  that  mode  of  slaking. 

7th.  Both  hydrauhc  and  common  mortar  yielded  better 
results  when  made  with  a  small  quantity  of  water  than  when 
made  thin. 

8th.  Mortar  made  in  the  mortar-mill  was  found  to  be  su- 
perior to  that  mixed  in  the  usual  way  with  a  hoe. 
9th.  Fresh  water  gave  better  results  than  salt  water. 
341.  Strength  and  Other  Properties  of  Portland  Cement. 

From  experiments  made  in  England  by  Mr.  Grant  on  the  re- 
sistance to  crushing  of  blocks  of  Portland  cement,  and  of 
Portland  cement  mortars,  the  following  results  are  deduced. 

1st.  The  strength  of  the  blocks  in  both  cases  increased  with 
time.  The  blocks  of  pure  cement  bearing  respectively  nearly 
4,000  lbs.  on  the  square  inch  after  three  months ;  over  5,000 
lbs.  at  six  months ;  and  nearly  6,000  lbs.  at  nine  months. 

2d.  The  strength  of  the  blocks  of  mortar  also  increased 
with  time  ;  but  decreased  as  the  volume  of  sand  used  was 
increased.  The  blocks  made  with  one  volume  of  sand  to  one 
of  cement  bore  about  2,500  lbs.  on  the  square  inch,  and  those 
made  of  six  volumes  of  sand  to  one  of  cement  959  lbs.  at 
the  end  of  three  months ;  whilst  those  made  of  one  volume 
of  sand  to  one  of  cement  bore  4,561  lbs.  on  the  square  inch 
at  the  end  of  nine  months,  and  those  made  of  six  volumes  of 
sand  to 'one  of  cement  bore  1,678  lbs.  on  the  square  inch  at 
the  end  of  the  same  period. 

From  numerous  experiments  made  by  Mr.  Grant  in  England, 
on  Portland  cement,  he  draws  the  following  conclusions  : — 

1st.  Portland  cement,  if  it  be  preserved  from  moisture, 
does  not,  like  Roman  cement,  lose  its  strength  by  being  kept 
in  casks  or  sacks,  but  rather  improves  by  age. 

2d.  The  longer  it  is  in  setting,  the  more  its  strength  in- 
creases. 

3d.  Yery  strong  Portland  cement  is  heavy,  of  a  blue-gray 
color,  and  sets  slowly.  Quick  setting  cement  has,  generally, 
too  large  a  portion  of  clay  in  its  composition,  is  brownish  in 
color,  and  turns  out  weak  if  not  useless. 

4th.  The  less  the  amount  of  water  in  working  the  cement 
up  the  better. 


118 


CIVIL  ENGINEEEING. 


6th.  It  is  of  the  greatest  importance  that  the  stones  or 
bricks,  with  wliich  Portland  cement  is  used,  should  be  thor- 
oughly soaked  with  water.  If  under  water,  in  a  quiescent 
state,  the  cement  will  be  stronger  than  out  of  water. 

6th.  Blocks  of  brickwork,  or  of  concrete,  made  with  Port- 
land cement,  if  kept  mider  w^ater  until  required  for  use, 
would  be  much  stronger  than  if  kept  dry. 

7th.  Salt  water  is  as  good  for  mixing  with  Portland  cement 
as  fresh  water. 

8th.  Poman  cement  is  very  ill  adapted  for  being  mixed 
with  sand. 

9th.  The  resistance  of  a  block  of  pure  Portland  cement  to 
extension  after  an  immersion  of  one  year  was  about  -ASO  lbs. 
on  the  square  inch  ;  whilst  the  resistances  of  blocks  made  of 
sand  and  cement,  after  the  same  period  of  immersion,  decreased 
with  the  quantity  of  sand  added.  Blocks  of  one  volume  of 
cement  in  paste  to  one  of  sand  giving  three-fourths  the  re- 
sistance of  those  of  pure  cement ;  and  those  of  one  volume 
of  cement  to  five  of  sand  giving  only  one-sixth  of  the  resist- 
ance of  blocks  of  pure  cement. 

10th.  Poman  cement  is  only  one-third  the  strength  of 
Portland  cement. — Proceedings  of  the  Institution  of  Civil 
Engineers^  Vol.  XXY.^  p.  66. 

342.  Concrete  and  Beton.  From  experiments  made  on 
concrete,  prepared  according  to  the  most  approved  process  in 
England,  by  Colonel  Pasley,  it  appears  that  this  material  is 
very  inferior  in  strength  to  good  brick,  and  the  weaker  kinds 
of  natural  stones. 

.  From  experiments  made  by  Colonel  Totten  on  beton,  the 
following  conclusions  are  drawn  : 

That  beton  made  of  a  mortar  composed  of  hydraulic 
cement,  common  lime,  and  sand,  or  of  a  mortar  of  hydraulic 
cement  and  sand,  without  lime,  was  the  stronger  as  the  quan- 
tity of  sand  was  the  smaller.  But  there  may  be  0.50  of  sand, 
and  0.25  of  common  lime,  without  sensible  deterioration ; 
and  as  much  as  1.00  of  sand,  and  0.25  of  lime,  without  great 
loss  of  strength. 

Beton  made  with  just  sufficient  mortar  to  fill  the  void  spaces 
between  the  fragments  of  stone  was  found  to  be  less  strong 
than  that  made  with  double  this  bulk  of  mortar.  But  Colonel 
Totten  remarks,  that  this  result  is  perhaps  attributable  to  the 
difficulty  of  causing  so  small  a  quantity  of  mortar  to  penetrate 
the  voids,  and  unite  all  the  fragments  perfectly,  in  experi- 
ments made  on  a  small  scale. 

The  strongest  beton  was  obtained  by  using  quite  small 


STRENGTH  OF  TIMBEE. 


119 


fraorments  of  brick,  and  the  weakest  from  small,  roanded. 

CD  ^  ' 

stone  gravel. 

A  iDeton  formed  by  pouring  grout  among  fragments  of 
stone,  or  brick,  was  inferior  in  strength  to  that  made  in  the 
usual  way  with  mortar. 

Comparing  the  strength  of  the  betons  on  which  the  experi- 
ments were  made,  which  were  eight  months  old  when  tried, 
with  that  of  a  sample  of  sound  red  sandstone  of  good  qiial- 
it}^,  it  appears  that  the  strongest  prisms  of  beton  were  only 
half  as  strong  as  the  sandstone. 

The  working  strain  on  masses  of  concrete,  brick,  and  rubble 
masonry  seldom  exceeds  in  structures  that  of  one-sixth  of  the 
crushing  weight  of  blocks  of  these  materials. 


lY. 

STRENGTH  OF  TIMBER. 

A  wide  range  of  experiments  has  been  made  on  the  resist- 
ance of  timber  to  compression,  extension,  and  a  transverse 
strain,  presenting  very  variable  results.  Among  the  most 
recent,  and  which  command  the  greatest  confidence  from  the 
ability  of  their  authors,  are  those  of  Professor  Barlow  and 
Mr.  Hodgkinson :  the  former  on  the  resistance  to  extension 
and  a  transverse  strain ;  -the  latter  on  that  to  compression. 


120 


CIVIL  ENGINEERING. 


The  following  Table,  taken  from  Vol.  V.  Professional 
Papers  of  the  English  Royal  Engineers.  No.  V.  Pe- 
marhs  and  Experiments  on  various  Woods,  give  some 
valuable  results  on  American  timber  subjected  to  a  strain 
parallel  to  the  fibre.  The  column  marked  C  gives  the  co- 
hesive strength. 


ce  and 
specific 

experiment. 

Mean 
Transverse 
Dimen- 

Trans- 

C. 

Extreme 
practical 
limit  of  C. 

No.   of  pie< 
average  i 
gravity. 

sions. 

o  .2 

'<u 
is 

t  ^ 

-2  c 

Remarks. 

o 
6 

B. 

D. 

^1 
ci  J2 

C  CO 

be 
C 

1 

1  Compute 
1   ing  wei 
square  i 

c 

S  o 
9  a 

II 
< 

A.SH,  AMERICAN. 

No.  9 
6:36 

9 
10 

11 
12 

13 
14 
15 
16 
17 

in. 
.6 

.605 

.608 
.585 
.615 
.593 
.588 
.597 
.62 

in. 
.607 
.613 

.615 

.575 

.615 

.61 

.61 

.6 

.62 

in. 
.3642 
.3708 

.3739 
.3363 
3782 
13617 
.3.586 
.3582 
.3844 

lbs. 

3088 
4051 

4142 
2528 
2528 
2675 
2:]87 
2580 
3473 

lbs. 

8478 
10925 

11077 
7517 
6()84 
7395 
6656 
7202 
9035 

5-9 

4-  7 

7-  9 

8-  9 
3-4 

5-  6 

lbs. 
4800 

Mr.  Barlow's  English  ash,  sp. 

gr.  760  C= 17:337. 
Mr.  Emerson's  do.,  C=6070. 

8330 

18 
19 

.588 

.598 

.3516 

2958 

8:319 

Crushed. 

ECH,  AMERICAN. 

20 
21 

22 

.61 
.607 

.61 
.603 

.3721 
.366 

3755 
3687 

10091 
10073 

if  there  be  rn 
no  crush-  § 
ing  force. 

Mr.  Barlow's  English  Beech, 

sp.  gr.  696  C  =  9912. 
Mr.  Tredgold's  do.,  sp.  gr.  696, 

C=2360. 

Crashed. 

23 
24 

.63 
.598. 

.612 
.612 

.3855 

3687 

9564 

Crushed. 

9512 

RCH.  BLACK 
MERICAN. 

No.  17 
645 

25 
26 
27 
28 
29 
30 

.603 
.612 
.628 
.627 
.601 
.595 

.575 

.59 

.56 

.597 

.597 

.597 

.3467 

.361 

.3516 

.3743 

.362:3 

.3552 

2567 
2086 
24(50 
282;3 
1904 
1848 

7404 
5778 
6!)96 
7512 
52,-)5 
8809 

5-6 
9-10 
5-8 

1  l-H 

:3-4 

4250 

Mr.  Emerson's  Birch,  4290. 

6959 

ELM,  CANADA. 

No.  29 
685 

45 
46 
47 
48 

.593 
.58 
.587 
.605 

.582 
.597 
.58 
.578 

.3451 
.3346 
.3404 
.3496 

3528  • 
47:54 
4500 
4263 

10223 
14148 
i:3219 
12193 

o 
o 

8000 

No.  26 
703 

49 
50 

.597 
.6 

.575 
.578 

.3432 
.3468 

4399 
4852 

12817 
13990 

« 

12765 

STRENGTH  OF  TIMBER. 


121 


HICKORY,  AMER- 
ICAN. 
<  *  » 

No.  of  piece  and 
average  specific 
gravity. 

No.  of  experiment. 

Mean 

Transverse 
Dimen- 
sions. 

Mean  area  of  Trans- 
verse Section, 

Breaking  weight. 

Computed  break- 
ing weight  per  p 
square  inch. 

Extreme 
practical 
limit  of  C, 

Remarks. 

Approximate 
proportion. 

B. 

D. 

No.  33 
856 

51 
52 
53 
54 
55 
56 

in. 

.583 
.583 
.566 
.593 
.595 
.595 

.63 

in. 
.618 
.61 
.59 
.575 
.595 
.588 

.608 

in. 

.3602 

.3556 

.3339 

.3409 

.354 

,3498 

.383 

lbs. 

3518 
4135 
3859 
4022 
3648 
3893 

4372 

lbs. 

97()(j 

11628 

115.57 

11798 

10305 

11192 

11420 

]l-13 

4-  5 
lu-l:i 
3-4 

5-  6 

10-13 

lbs. 

8000 

No.  30 
854 

57 

11095 

OAK,  WHITE  AMERI- 
CAN. 

No.  45 
716 

No.  49 
600 

80 
81 

82 
83 
84 
85 
86 

87 

.59 
.598 

.587 

.598 

.59 

.597 

.588 

.608 

.595 
.595 

.597 
.542 
.5^5 
.5i>8 
.607 

.627 

.351  4414 
.356  4890 

.3504  *  3201 
.3241  2958 
.3451  2958 
.357  3088 
.3569  2755 

.3812  3523 

1 

12547 
13736 

9107 
9126 
8578 
8646 
7719 

9241 

9-10 

6-  7 

4-5 
4-5 
4-5 

7-  8 

6000 

Mr.  Barlow's  Canadian  Oak, 
sp,  gr.  872,  C  =  11428, 

No  45 
716 

9750 

OAK,  BASKET, 
AMERICAN. 

No.  51 
937 

88 
89 
90 
91 
92 
93 
94 

.582 
.607 
.628 
.542 
.585 
.608 
.602 

.566 

.573 

.583 

.61 

.697 

.547 

.603 

.3294 
.3478 
.3661 
.3306 
.4077 
.3325 
.363 

10710 

10684 
9158 

10759 
8253 
8297 

11374 

6-  7 

7-  9 

5-6 
7-9 

6000 

9891 

OAK,  ENGLISH,  SAWN 
AND  SEASONED. 

No.  62 

asi 

120 
121 
122 
123 

.605 
.595 
.605 
.593 

.598 

.6 

,6 

.585 

.605 
.605 
.605 
.6 

.61 

,605 

,59 

,6 

.366 
.3599 
.36() 
.3558 

,3647 

.363 

.334 

.351 

3376 
2315 
2725 
2315 

3213 
2963 
2408 

2408 

9224 
6432 
7445 
6506 

8809 
8162 
6802 

6860 

3-4 
7-10 

7-11 
7-11 
5-6 

5500 

"River  and  green,"  beats 
sawn  and  seasoned,"  with 
reference  to  both  S  and  G, 
The  uninjured  state  of  the 
grain  has,  I  apprehend,  more 
to  do  with  the  strength  than 
the  condition  as  to  the  dry- 
ness. 

Tredgold's  English  Oak  sp,  gr. 
830,  C=3960. 

No.  63 
836 

124 
125 
126 

127 

7530 

ERI- 

No.  65 
1230 

128 

.598 

.6 

.3588 

3528 

9832 

No.  67 
1141 

129 
130 

.603 
.59 

.6 

.608 

.3618 
.3587 

2688 
4113 

7429 
11466 

;ords  1( 

4000 

W 

i 

No.  68 
1140 

131 

132 
133 

.57 

.627 

.597 

.603 
.577 
.573 

111. 

3111 
2128 
2128 

9051 
588;3 
6222 

G 

8314 

122 


CIVTL  ENGINEERING. 


;e  and 
specific 

•iment. 

Mean 
Transverse 
Dimen- 

t 

H  . 

4i 

C. 

Extreme 
practical 
limit  of  C. 

No.  of  piec 
average  i 
gravity. 

sions. 

<4-l  i3 

0.2 

o  ir-. 

.-1 

^ 

O  Pa 

3.2 

Bemarks. 

d 
IZi 

B. 

D. 

Mean  ar 
verse  S( 

to 
a 

1 
i-i 
tt 

Compute 
ing  wei 
square  i 

Approxir 
proport 

No.  76 

568 

134 

135 
136 

in. 
.585 

.587 
.582 

in. 
.607 

.6 

.592 

in. 
.355 

.3522 
.3445 

lbs. 
1528 

1925 
1868 

lbs. 
4304 

5467 
5422 

2-3 

2-  3 

3-  6 

lbs. 

Mr.  Barlow's  New  England 
Pir,  sp.  gr.  553,  C=9947. 

PINE,  RED 
CAN. 

No.  78 

639 

137 

138 
139 

.587 

.587 
.588 

.59 

.59 
.64 

.3463 

.3463 
.3763 

1748 

1975 
2449 

5047 

5703 
6508 

3-5 

23 

3000 

Mr.  Tredgold's  Yellow  Ameri- 
can, sp.  gr.  460,  C=3900. 

5408 

PINE.  WHITE 
AMERICAN. 

No.  71 
450 

140 
141 
142 
143 
144 
145 

.62 

.607 

.63 

.635 

.607 

.627 

.625 
.602 
.627 
.635 
.625 
.617 

.3875 

.3654 

.395 

.3968 

.3793 

.3868 

3674 
3295 
3936 
3520 
3418 
3640 

5-7 
2-3 
5-6 
5-7 

10-  J  -i 

2200 

3580 

343.  Resistance  to  Tensile  Strain.  The  following  table 
exhibits  the  specific  gravity,  and  the  mean  resistance  per 
square  inch  of  various  kinds  of  timber,  from  the  experiments 
of  Prof.  Barlow. 

The  working  strain  on  beams  subjected  to  extension  should 
not  exceed  y\  of  the  rupturing  strain  in  permanent  structures, 
but  for  tempoi-aiy  purposes,  like  scaffolding,  cfec,  it  may  be 
placed  at  ^th  the  rupturing  strain  with  safety. 

But  few  direct  experiments  have  been  made  upon  the 
elongations  of  timber  from  a  strain  in  the  direction  of  the 
fibres.  From. some  made  in  France  by  MM.  Miuard  and 
Desormes,  it  would  a])pear  that  bars  of  oak  having  a  sectional 
area  of  one  square  inch  will  be  elongated  .OOllTG  of  their 
length  by  a  strain  of  one  ton. 

344.  Resistance  to  Compressive  Strains.  The  follow- 
ing Table  exhibits  the  results  ol)taiiied  hy  Mr.  Ilodgkinson 
from  experiments  on  short  cylinders  of  timber  with  fiat  ends. 
The  diameter  of  each  cylinder  was  one  inch,  and  its  height  two 
inches.  The  results,  in  the  first  column,  are  a  mean  from 
about  three  experiments  on  timber  moderately  dry,  being 
such  as  is  used  for  making  models  for  castings ;  those  in  the 
second  column  were  obtained  in  a  like  manner,  from  similar 


STRENGTH  OF  TIMBEK. 


123 


DESCRIPTION  OF  TmBER. 


Ash  {English)  

Beech,  do  

Box  

Deal  {Christiania)  

Do.  {Memel)  

Elm  

Fir  (:NeiD  England)  

Do.  {Riga)  

Do.  {Mar  Forest)  

Larch  {8coU7i).  

Locust  

Mahogany  

Norway  spars  

Oak  {Englisli)  |  ^^^"^ 

Do.  {African)  

Do.  {Adriatic)  

Do.  {Canadian)  

Do.  {Dantzic)  

Pear  

Poon  

Pine  {pitch)  

Do.  {red)  

Teak  


Spec.  grav. 


0.760 

17000 

0.700 

11500 

LOOO 

20000 

0.680 

11000 

0.590 

11000 

0.540 

5780 

0.550 

12000 

0.750 

12600 

0.700 

12000 

0.540 

7000 

0.950 

20580 

0.637 

8000 

0.580 

12000 

0.700 

9000 

0.900 

15000 

0.980 

14400 

0.990 

14000 

0.872 

12000 

0.760 

14500 

0.646 

9800 

0.600 

14000 

0.660 

10500 

0.660 

10000 

0.750 

15000 

specimens,  wliich  were  turned  and  kept  dry  in  a  warm  place 
two  months  longer.  A  comparison  of  the  results  in  the  two 
columns  shows  the  effect  of  drying  on  the  strength  of  tim- 
ber ;  wet  timber  not  having  half  the  strength  of  the  same 
when  dry.  The  circumstances  of  rupture  were  the  same  as 
already  stated  in  the  general  observations  under  this  head ; 
the  height  of  the  wedge  which  would  slide  off  in  timber 
being  about  half  the  diameter  or  thickness  of  the  specimen 
crushed. 

345.  Resistance  of  Square  Pillars.  Mr.  Hodgkinson  has 
made  a  number  of  invaluable  experiments  on  the  strength  of 
pillars  of  timber,  and  of  columns  of  iron  and  steel,  and  from 
them  has  deduced  formulas  for  calculating  the  pressure 
which  they  will  support  before  breaking.  The  experiments 
on  timber  were  made  on  pillars  with  flat  ends.  The  follow- 
ing are  the  formulae  from  which  their  strength  may  be  esti- 
mated. 

Calling  the  breaking  weight  in  lbs.,  W. 

"     the  side  of  the  square  base  in  inches,  d. 
"     the  length  of  the  pillar  in  feet,  I. 


124 


CIVIL  ENGINEERING. 


DESCIUPTION  OF  WOOD. 


Alder  , . . . . 

Ash  

Bay  wood  

Beech  

Birch  {American)  

Do.  {English)  

Cedar  

Crab  '.  

Red  deal  

White  deal  

Elder  

Elm  

Fir  {spruce)  

Hornbeam  

Mahogany  

Oak  {Quebec)  , 

Do.  {English)  

Do.  {Dantzic,  very  dry)  , 

Pine  {pitch)  

Do.  {yellow^  full  of  turpentine) 

Do.  {red)  

Poplar  

Plum  {wet)  

Do.  {dry)  

Sycamore  

Teak  

Larch  {fallen  two  months)  

Walnut  

Willow  


strength  per  square  inch, 
in  lbs. 


C831 

6960 

8683 

9363 

7518 

7518 

7733 

19363 

3297 

11663 

3297 

6402 

5074 

5863 

6499 

7148 

5748 

6586 

6781 

7293 

7451 

9973 

~ 

10331 

6499 

6819 

4533 

7289 

8198 

8198 

4231 

5982 

6484 

10058 

7731 

6790 

6790 

5375 

5445 

5395 

7518 

3107 

5124 

3G54 

8241 

to  1049 

7082 

12101 

3201 

5568 

6063 

7227 

2898 

6128 

•  Then  for  long  columns  of  oak,  in  which  the  side  of  the 
square  base  is  less  than  -gi^  th  the  height  of  the  column ; 

TF  ^  24542  ^, 

and  for  red  deal,  ^4 

W=  1Y511  -f- 
For  shorter  pillars,  where  the  ratio  between  their  thickness 
and  height  is  such  that  they  still  yield  by  bending,  the 
strength  is  estimated  by  the  following  formula  : 

Calling  the  weight  calculated  from  either  of  the  preceding 
formulae,  W. 

Calling  the  crushing  weight,  as  estimated  from  the  pro- 
ceeding table,  TP. 

Calling  the  breaking  weight  in  lbs.,  W" , 
Then  the  formula  for  the  strength  is 

TF"  =  -JnEL- 
1F+  f  W 


STRENGTH  OF  TIMBEE. 


125 


In  each  of  the  preceding  formulae  d  must  be  taken  in 
inches,  and  I  in  feet. 

The  same  rule  is  followed  in  proportioning  the  rupturing 
to  the  working  strain  in  timber  subjected  to  compression  as  in 
timber  subjected  to  extension. 

346.  Resistance  to  Transverse  Strains.  As  timber, 
from  the  purposes  to  which  it  is  applied,  is  for  the  most  part 
exposed  to  a  transverse  strain,  the  far  greater  number  of  ex- 
periments liave  been  made  to  ascertain  the  relations  between 
the  strain,  the  deflection  caused  by  it,  and  the  linear  dimen- 
sions of  the  piece  subjected  to  the  strain.  These  relations  have 
been  made  the  subject  of  mathematical  investigations,  found- 
ed upon  data  derived  from  experiment,  which  will  be  given 
in  the  Appendix.  The  following  table  exhibits  the  results  of 
experiments  made  upon  beams  having  a  rectangular  sectional 
area,  which  were  laid  horizontally  upon  supports  at  their  ends, 
and  subjected  to  a  strain  applied  at  the  middle  point  between 
the  supports,  in  a  vertical  direction. 

For  a  more  convenient  application  of  the  formulae  to  the 
results  of  the  experiments,  the  notation  adopted  in  the  pre- 
ceding Art.  will  be  here  given. 

Call  the  transverse  force  necessary  to  break  the  beam  in 
lbs.,  W. 

Call  the  distance  between  the  supports  in  inches,  I. 
"    the  horizontal  breadth  of  the  sectional   area  in 
inches,  h. 

Call  the  vertical  depth  of  the  sectional  area  in  inches,  d. 
"    the  deflection  arising  from  a  weight  w  in  inches,/! 

Table  of  Experiments  with  the  foregoing  Notation. 


Values 

Values 

Value 

Value 

Value 

Value 

DESCKIPTION  OF  WOOD. 

Specific 

of 

of 

of 

of 

of 

of 

Authors  of 

grav. 

I. 

h. 

d. 

/. 

w. 

W. 

experiments. 

Inches. 

Inches. 

Inches. 

Inches. 

lbs. 

lbs. 

Oak  {English)  

.934 

.s4 

2 

2 

1.280 

200 

637 

Prof.  Barlow. 

.872 

84 

2 

2 

1.080 

225 

673 

84 

2 

0.931 

150 

Oak  {English)  

30 

1 

1 

0.5 

137 

Tredgold, 

White   spruce  {Ca7ia- 

.465 

24 

1 

1 

G.5 

180 

285 

White  pine  {American) 

.455 

85.2 

2.75 

5.55 

0.177 

777 

5189 

Lieut.  Brown. 

Black  spruce,  do. 

.490 

8.5.2 

2.75 

5.55 

0.177 

892 

5646 

Southern  pine,  do. 

.872 

85.2 

2.75 

5.54 

0.177 

1175 

9237 

126  CIVIL  ENGINEERING. 

The  following  Table,  tahen  from  Vol.  V.  Professional 
Papers  of  the  English  Royal  Engineers.  No.  V.  Re- 
marks and  Exjperiments  on  various  Woods,  gives  the  value 

of  S,  in  the  formula  S  =  J^^,for  transverse  strains,  in 


which  I,  the  length  of  the  pieces  subjected  to  experiment, 
was  from  five  to  six  feet  ^  the  distance  between  the  points 
of  support  four  feet ;  the  ends  of  the  pieces  not  confined. 


"S 

a> 

Transverse 
dimensions. 

1          Ultimate  deflection. 

flection  = 

deflections 
iniform. 

lections. 

1 

ed  succes- 

No.  of  experim 

Specific  gravil 

Mean  depth. 

Mean  breadth. 

Breaking  weig 

Weight  giving  a  dei 
1-100  length 

Weight  at  which  the  i 
ceased  to  be  at  all  u 

Corresponding  defl 

a 

o 

^  II 

O  02 
> 

No.  of  weights  appli 
sively. 

Detail  Remarks. 

;H,  AMERICAN. 

7 

8 
9 

618 

580 
636 

in. 
1.98 

2.0 
2.0 

in. 
1.98 

1.85 
1.85 

lbs. 
1101 

803 
1017 

in. 
2.0 

1.8 
3.0 

lbs. 
390 

298 
271 

lbs. 
642 

478 
534 

in. 
.8 

.8 
.925 

1702 

13C0 
1649 

21 

16 
19 

Good  specimen;  gave 
warning  at  1017  lbs., 
then  fell  rapidly  and 
broke  at  1101  lbs. 

Tolerable  specimen ; 
gave  warning  grad- 
ually at  751  lbs. 

Do.  as  No.  8. 

131  1 

<  L 

611 

1550 

ECH,  AMERI- 
CAN. 

10 

11 

12 

782 

788 
765 

2.05 

2.0 

1.98 

1.98 

2.0 
2.0 

1241 

1073 
1157 

2.7 

1.9 
2.6 

428 

416 

428 

697 

642 

534 

.85 

.775 
.625 

1790 

1609 
1770 

24 

20 
24 

Tolerable      specimen : 
gave  warning  at  603 
lbs. 

Good  specimen;  gave 
warning  at  1017  lbs. 

Do.  broke  well  and 
gradually. 

m 

778 

1723 

BIRCH,    BLACK  AMERI- 
CAN. 

13 

14 
15 
10 
17 

764 

646 
720 
634 
645 

2.0 

2.0 
2.0 
2.0 
2.0 

2.0 

1.98 
2.0 
2.0 
2.0 

1521 

1297 
1017 
1129 
1185 

1.7 

2.8 
2.7 
2.5 
3.3 

540 

390 
487 
536 
470 

1241 

697 
642 
803 
642 

1.275 

.875 
1.1 

1.17 
1.1 

2282 

1965 
1525 
1693 
1777 

30 

26 
24 
25 
25 

Very  good   specimen ; 
warning  at  1270  lbs., 
broke    suddenly  at 
1521. 

Good  specimen;  broke 
suddenly  at  1297  lbs. 

Do.,  broke  with  a  long 
scarf  and  gradually. 

Do.,  broke  well,  but  with 
little  warning. 

Do.  Do.  Do. 

682 

1848 

All  taken  from  the  same 
piece. 

§  . 

20 
27 
28 
29 

703 
700 
712 
6S5 

2.046 
2.05 

2.037 
2.03 

2.008 
2.037 
2.03 
2.025 

1377 
1205 
1321 
1265 

3.1 
2.5 
3.5 
3.5 

230 
486 
4»3 
451 

761 
649 
673 
621 

0.9 
1.5 

0.8 
0.74 

1906 
1799 
1891 
1819 

38 
35 
30 
35 

The  great  uniformity  of 
texture  in  this  wood 
presented  no  irregu- 
larities for  comment 

700 

1669 

during  straining. 

6TEENGTH  OF  TIMBER. 


127 


II 

m 

0) 

Transverse 

d 

•li 

i 

o 
o 

dimensions. 

d 
.2 

3ectio 

a>  o 

formi 

g 

11 

o 

:  experim 

lific  gravil 

p. 

-c 

king  weig 

ate  deflec 

ng  a  del 
00  length 

hich  the  ( 
be  at  all  u 

« 
'O 

60 

.3 

a 

cl 

M 
O 

OQ^Il 

hts  applii 
sively. 

Detail  Hemarks. 

No.  oJ 

Spec 

Mean  d( 

Mean  br( 

Brea 

Ultim 

3ight  givi 
1-1 

sight  at  w 
?.eased  to  1 

Correspo: 

^  11 

O  CO 

CD 

> 

No.  of  weig 

in. 

in. 

lbs. 

in. 

lbs. 

lbs. 

in. 

30 

854 

2.0 

2.0 

1330 

3.0 

390 

857 

1.75 

1995 

27 

.        ,  J.  -.1, 
(jood  piece,  but  with 

a  small  knot  12  inches 

from    centre ;  gave 

<; 

warning  at  1129  lbs., 
broke  at  1330  equally 

O 

at  the  knot  and  cen- 

, AME 

tre. 

31 

838 

1.98 

1.97 

857 

1.2 

390 

590 

.7 

1332 

16 

Indifferent  specimen, 

two-fifths  sap. 

o 

32 

866 

2.03 

2.0 

1270 

1.9 

481 

910 

.975 

1607 

29 

Good  specimen,  warn- 

ing at  642  lbs. 

33 

856 

2.0 

1.98 

1157 

3.0 

405 

590 

.7 

1753 

23 

Do.    warning  at  1129 

o 

lbs.,  broke  well  and 
gradually. 

s 

871 

1672 

45 

716 

1.93 

1.92 

1300 

2.0 

370 

751 

1.025 

2181 

30 

Good  specimen,  warn- 

ino-  nf  1240  Ihd 

46 

1.95 

1.92 

963 

2.0 

331 

478 

.75 

1582 

18 

Broke  soon  at  a  knot  \ 
no    specific  gravity 
mentiont'd,  46  and  47 
having  been  at  first 

sui^posed  to  be  too 

unsatisfactory  \  they 

were,    however,  re- 
corded, as  No.  50  did 

not  give  a  very  much 

H 

better  result. 

47 

1.93 

1.93 

803 

2.0 

426 

642 

.775 

1342 

15 

W 

48 

666 

2.05 

2.03 

1120 

1.9 

286 

590 

1.0 

1580 

22 

Good-looking  specimen. 

but  slightly  tainted 

with  dry-rot ;  broke 

with  little  warning. 

49 

600 

2.00 

2.03 

1297 

2.0 

349 

478 

.65 

1916 

26 

Do.  Do.   broke  with  a 

o 

long  scarf. 

50 

600 

2.17 

0.86 

534 

1.8 

211 

366 

.9 

1582 

10 

A  slab  specimen  from 
48. 

645 

1699 

51 

987 

1.83 

1.69 

910 

3.5 

244 

478 

1.15 

1929 

17 

i  air  specimen  ;  warn- 

A.MER 

ing  at  about  400  lbs. ; 
broke    with  a  long 

scarf. 

52 

947 

1.81 

1.68 

697 

3.6 

207 

310 

1.6 

1339 

13 

Broke  at  a  large  worm- 

hole,  to  which  this 

pa5  ■ 

wood  seems  to  be  sub- 

ject. 

< 

53 

937 

1.8 

1.6 

803 

3.0 

478 

1.3 

1859 

15 

Do.    These  three  speci- 

mens were  all  from 

the  same  log. 

o 

940 

1709 

♦ 


128 


CIVIL  ENGINEERING. 


1120 
1230 
1121 
1141 
1140 


1209 
1160 


Transverse 
dimensions. 


2.029 
2.025 
2.046 
2.029 
2.042 


2.025 


in. 

2.004 
2.015 
2.0.39 
1.99 
2.02:3 


2.017 


lbs. 

1041 
1433 
1265 
1489 
873 


1209 


3GS 


lbs. 

313 

565 
313 
313 
257 


453 


.58 


„  II 
o  ai 


1513 

2080 
1T80 
2181 


1756 
1862 


31 


Detail  Remarks. 


Evidently  a  bad  speci- 
men, though  it  looked 
weU. 


422 


4.50 
432 

480 
480 
453 

453 


2.0 
2.0 

2.008 
1.98 
2.0 


2.0 


2.0 
2.0 

1.99 
1.97 
1.99 


910 


910 
910 

1041 
985 
1041 


1.8 


316 


534 


590 
590. 


.85 


1351 


1365 
1365 

1557 
1531 
1569 


1456 


Good  clean  specimen ; 

broke  short  without 

warning. 
Do.  Do. 

Do.   All  from  the  same 
log. 

Do.  1  No  remarks  made 
Do.  >-  at  the  time  of 
Do.  )  experiment. 


re  ,568 


77  656 

78  639 


2.0 


2.0 
2.0 


2.0 
2.0 


1157 


1420 
1300 


2.1 


369 


642 


590 
963 


.6 
1.125 


1753 


2130 
1950 


1944 


Snapi)ed  at  the  centre  ; 
though  there  was  a 
knot  8  inches  from  it. 

Good  clean  specimen. 

Do.,  but  broke  remark- 
ably short,  and  with- 
out warning. 


Deflection  of  Wooden  Beams.  Professor  W.  A.  TsTorton, 
of  the  Scientific  School  of  Yale  College,  made  a  careful  series 
of  experiments,  to  test  the  practical  accuracy  of  the  formula 
derived  from  the  generally-received  theory  of  the  deflection 
of  beams  of  a  rectangular  cross-section,  arising  from  a  weight 
acting  at  the  middle  point  of  the  beam  resting  on  two  sup- 
ports, its  axis  being  horizontal. 

This  formula  is  :  f^iUj^  ^  ^3 ;  in  which 

P  is  the  applied  j)i'essure  ;  /J  the  deflection  due  to  P  ;  the 
modulus  of  elasticity  of  the  material ;  5,  the  breadth ;  the 


STKENGTH  OF  TIMBER. 


129 


depth  ;  and  the  distance  between  the  points  of  support  of 
the  beam ;  and      a  constant  to  be  derived  from  experiment. 

From  this  formula,  if  accurate,  the  amount  of  deflection 
should  vary  directly  as  the  pressure  and  cube  of  the  length, 
and  inversely  as  the  breadth  and  cube  of  the  depth  ;  but  from 
Prof.  Norton's  experiments  it  appears : — 

1.  Tliat  the  deflection  varies  approximately  as  the  pressures, 
but  rather  increasing  according  to  a  less  rapid  law. 

2.  That,  although  the  deflections  are  not  uniformly  in- 
versely as  the  breadth,  still  the  variation  from  this  law  is  but 
slight. 

3.  That,  except  in  "  beams  whose  length  bore  a  high  propor- 
tion to  their  depth,"  the  law  indicated,  that  the  deflections  are 
inversely  proportional  to  the  cubes,  is  far  from  being  accurate. 
In  other  cases  it  "  decreases  according  to  a  less  rapid  law  than 
the  inverse  cube  of  the  depth." 

4.  The  experiments  also  show,  that  the  law,  that  the  deflec- 
tion is  directly  proportional  to  the  cube  of  the  length,  also 
fails. 

From  these  experiments  Prof.  N"orton  says : — 
"  We  may  conclude,  from  these  results,  that  the  deflection 
increases  according  to  a  less  rapid  law  than  the  cube  of  the 
length  of  the  stick.  We  have  already  seen  that  it  decreases 
in  a  less  rapid  proportion  than  the  inverse  cube  of  the  depth. 
It  follows,  therefore,  that  the  true  formula  for  the  deflection 
probably  contains  at  least  one  additional  term,  which  varies 
less  rapidly  than  as  the  cube  of  the  length  directly  and  the 
cube  of  the  depth  inversely  ;  or  in  otliler  words,  contains  I  in 
the  numerator,  and  d  in  the  denominator,  each  raised  to  a 
lower  power  than  the  cube." 

"  Further,  it  would  seem,  then,  that  the  true  theory  of  de- 
flection conducts  to  the  following- formula,  in  the  special  case 
of  a  beam  resting  on  two  supports  and  loaded  in  the  middle. 

The  following  table  gives  the  values  of  E  for  white  pine, 
and  the  calculated  values  of  the  constant  O. 

"  The  general  formula  applicable  to  white  pine  sticks  of  the 
general  quality  used  in  these  experiments  will  be  obtained  by 
taking  the  mean  of  the  several  values  of  E  and  O  given  in  the 
above  table.  To  test  the  theoretical  formula  we  have  obtained 
w^e  will  take  the  mean  values  of  E  and  for  the  second  set 
of  sticks,  given  at  the  bottom  of  the  fourth  and  fifth  columns, 
viz. :  ^=1,427,965  pounds,  and  {7=  0.0000094  We  thus 
have 

9 


130  CIVIL  ENGINEEEING. 

/=0.0000094-3^+5;^g„-^ 
or,  taking  P=100  lbs., 

/=0.00094-^  +  5^jg^.." 

The  general  formula  for  the  deflection  may  also  take  the 
following  form : 


TABLE. 


sticks. 

DifE.  of  _  Extreme  Pressures. 

DiS.  of  Intermediate 
Pressures. 

Set  No.  1. 

E. 

C. 

JS. 

C. 

I. 

6. 

1,3.59.500  lbs. 
1,.566,809  " 
1,584,820  " 
1,552,000  " 
1,481,800  " 
1,5U8,986 

1,277,729  lbs. 
1,295,984  " 
1,558,900  " 
1,501,822  " 
1,423,609  " 

0.0000108 
0.0000100 
0.0000087 
0.0000140 
0.0000108 
0.0000108 

0.0000084 

0.0000089 
0.0000110 
0.0000084 
0.0000092 

1,308,430  lbs. 
1,579,960  " 
1,560,800  " 
1,501,200 
1,423,600  " 
1,474,798  " 

1,254,000  lbs. 
1,315,000  " 
1,542,860  " 
1,600,000  " 
1,427,965  " 

0.0000082 
0.0000095 
0.0000078 
0.0000127 
0.0000084 
0.0000080  . 

0.0000080 
0. 0000088 
0.0000107 
0.0000100 
0.0000094 

ft.  ft. 
2,  or  4 
2 
4 

2,  or  4 
2,  or  4 

Mea 

in.  in. 

2 

2,  or  3 
2,  or  3 
4 
2 

in.  in. 
1 

3,  or  2 
3,  or  2 
2 
2 

Set  No.  2. 

ft.  ft. 

2,  or  4 
2,  or  4 
2,  or  4 
2,  or  4 

Mear 

in. 

3 
2 
4 
2 

in. 

2 
3 
2 
2 

347.  Resistance  to  Detrusion.  From  the  experiments  of 
Prof.  Barlow,  it  appears  that  the  resistance  offered  by  the 
lateral  adhesion  of  tiie  fibres  of  fir,  to  a  force  acting  in  a 
direction  parallel  to  the  fibres,  may  be  estimated  at  592  lbs. 
per  square  inch. 

Mr.  Tredgold  gives  the  following  as  the  results  of  experi- 
ments on  the  resistance  offered  b}^  adhesion  to  a  force  applied 
perpendicularly  to  the  fibres  to  tear  them  asunder. 

Oak  231 G  lbs.  per  square  inch. 

Poplar  1783  " 

Larch,  970  to  1700     "  " 


STRENGTH  OF  CAST-IRON 


131 


V. 

STRENGTH  OF  CAST-IRON. 

The  most  recent  experiments  on  the  strength  of  this  ma- 
terial are  those  of  Mr.  Hoclgkinson.  Those,  particularly, 
made  by  him  on  the  subject  of  the  strength  of  columns,' 
and  the  most  suitable  form  of  cast-iron  beams  to  sustain  a 
transversal  strain,  have  supplied  the  engineer  and  architect 
with  the  most  valuable  guide  in  adapting  this  material  to 
the  various  purposes  of  structures. 

348.  Resistance  to  Extension. — From  a  few  experiments 
made  by  Mr.  Kennie  and  Captain  Brown,  the  tensile  strength 
of  cast  iron  varies  from  7  to  9  tons  per  square  inch. 

The  experiments  of  Mr.  Hodgkinson  upon  both  hot  and 
cold  blast  iron  give  the  tensile  strength  from  6  to  9f  tons  per 
square  inch. 

From  some  experiments  made  on  American  cast  iron,  under 
the  direction  of  the  Franklin  Institute,  the  mean  tensile 
strength  is  20834  lbs.,  or  9-J-  tons  per  square  inch. 

349.  Resistance  to  Compressive  Strain. — The  general 
circumstances  attending  the  rupture  of  this  material  by  com- 
pression, drawn  from  the  experiments  of  Mr.  Hodgkinson, 
have  already  been  given.  The  angle  of  the  wedge  resulting 
from  the  rupture  is  about  55°. 

The  mean  crushing  weight  derived  from  experiments  upon 
short  cylinders  of  hot  blast  iron  was  121,685  lbs.,  or  54  tons 
6^  cwt.  per  square  inch. 

That  on  short  prisms  of  the  same,  with  square  bases, 
100,738  lbs.,  or  44  tons  19^  cwt.  per  square  inch. 

That  on  short  cylinders  of  cold  blast  iron  was  125,403  lbs., 
or  55  tons  19^  cwt.  per  square  inch. 

That  on  short  prisms  of  the  same,  having  other  regular 
figures  for  their  bases,  was  100,631  Ihs.,  or  44  tons  18^  cwt. 
per  square  inch. 

Mr.  Hodgkinson  remarks  with  respect  to  the  forms  of  base 
differing  from  the  circle:  "In  the  other  forms  the  difference 
of  strength  is  but  little ;  and  therefore  we  may  perhaps  admit 
that  difference  of  form  of  section  has  no  influence  upon  the 
power  of  a  short  prism  to  bear  a  crushing  force." 

In  remarking  on  the  circumstances  attending  the  rupture, 
Mr.  Hodgkinson  further  observes  :  "  We  may  assume,  there- 
fore, without  assignable  error,  that  in  the  crushing  of  short 


132 


CIVIL  ENGINEEEING. 


iron  prisms  of  various  forms,  longer  than  the  wedge,  the  angle 
of  fracture  will  be  the  same.  This  simple  assumption,  if  ad- 
mitted, would  prove  at  once,  not  only  in  this  material,  but  in 
others  which  break  in  the  same  manner,  the  proportionality  of 
the  crushing  force  in  different  forms  to  the  area ;  since  the 
area  of  fracture  would  always  be  equal  to  the  direct  trans- 
verse area  multiplied  by  a  constant  quantity  dependent  upon 
the  material." 


Table  exhibiting  the  Ratio  oj^  the  Tensile  to  the  Comjpres- 
sive  Forces  in  Cast  Iron,  from  Mr.  Hodgldnsovb  s  Exjperi- 
ments. 


DESCRIPTION  OF  METAL. 

Compressive  force 
per  square  inch. 

Tensile  force  per 
square  inch. 

Ratio. 

Devon  iron, 

No.  3.  Hot  blast 

145,435 

21,007 

6.038  : 

1 

Buff  ery  iron, 

No.  1.  Hot  blast 

86,397 

13,434 

6.431  : 

1 

Do. 

"     Cold  blast 

93,385 

17,466 

5.346  : 

1 

Coed-Talen  iron, 

No.  2.  Hot  blast 

82,734 

16,676 

4.901  : 

1 

Do. 

"     Cold  blast 

81,770 

18.855 

4  337  : 

1 

Carron  iron, 

No.  2.  Hot  blast 

108,540 

13,505 

8  037  : 

1 

Do. 

"     Cold  blast 

106,375 

16,083 

6.376  : 

1 

Carron  iron, 

No.  3.  Hot  blast 

133,440 

17,755 

7.515  : 

1 

Do. 

"     Cold  blast 

115.442 

14,200 

8.129  : 

1 

350.  Resistance  of  Cylindrical  Columns.  The  experi- 
ments under  this  head  were  made  upon  solid  and  hollow  col- 
UQins,  both  ends  of  which  were  either  flat  or  rounded,  fixed  or 
loose,  or  one  end  flat  and  the  other  rounded.  In  tlie  case  of 
columns  with  rounded  ends,  the  pressure  was  applied  in  the 
direction  of  the  axis  of  the  column. 

The  following  extracts  are  made  from  Dr.  Ilodgkinson's 
paper  on  this  subject,  published  in  the  Report  of  the  British 
Association  (9/1840. 

"  1st.  In  all  long  pillars  of  the  same  dimensions,  the  resist- 
ance to  crushing  by  flexure  is  about  three  times  greater  when 
the  ends  of  tlie  pillars  are  flat  than  when  they  are  rounded. 

"  2d.  The  strength  of  a  pillar,  with  one  end  rounded  and 
the  other  flat,  is  the  arithmetical  mean  between  that  of  a 
pillar  of  the  same  dimensions  with  both  ends  round,  and  one 
with  both  ends  flat.  Thus,  of  three  cylindrical  pillars,  all  of 
the  same  length  and  diameter,  the  first  having  both  its  ends 
rounded,  the  second  with  one  end  rounded  and  one  fiat,  and 
the  third  with  both  ends  fiat,  the  strengths  are  as  1,  2,  3, 
nearly. 


6TKENGTH  OF  CAST-IEON. 


133 


"  3d.  A  long,  uniform,  cast-iron  pillar,  with  its  ends  firmly 
fixed,  whether  by  means  of  disks  or  otherwise,  has  the  same 
power  to  resist  breaking  as  a  pillar  of  the  same  diameter,  and 
half  the  length,  with  the  ends  rounded  or  turned  so  that  the 
force  would  pass  through  the  axis. 

"  4th.  The  experiments  show  that  some  additional  strength 
is  given  to  a  pillar  by  enlarging  its  diameter  in  the  middle 
part ;  this  increase  does  not,  however,  appear  to  be  more  than 
one  seventh  or  one  eighth  of  the  breaking  weight. 

"  5th.  The  index  of  the  power  of  the  diameter  to  which  the 
strength  of  long  pillars  with  rounded  ends  is  proportional,  is 
3.76  nearly,  and  3.55  in  those  with  flat  ends,  as  appeared  from 
the  results  of  a  great  number  of  experiments  ;  or  the  strength 
of  both  may  be  taken  as  the  3.6  power  of  the  diameter 
nearly. 

"  6th.  In  pillars  of  the  same  thickness,  the  strength  is  in- 
versely proportional  to  the  1.7  power  of  the  length  nearly. 
"  Thus  the  strength  of  a  solid  pillar  with  rounded  ends,  the 

^3-6 

diameter  of  which  is    and  the  length  Z,  is  as 

Z 

"  The  absolute  strength  of  solid  pillars,  as  appeared  from 
the  experiments,  are  nearly  as  below. 
"  In  pillars  with  rounded  ends, 

Strength  in  tons  =  14.9  — . 

"  In  pillars  with  flat  ends, 

Strength  in  tons  =  44.16  ^  • 

In  hollow  pillars  nearly  the  same  laws  were  found  to  ob- 
tain ;  thus,  if  I)  and  d  be  the  external  and  internal  diameters 
of  a  pillar  whose  length  is  Z,  the  strength  of  a  hollow  cylinder 
of  which  the  ends  were  movable  (as  in  the  connecting-rod  of 
a  steam-engine)  would  be  expressed  by  the  formula  below. 

Strength  in  tons=  13  -p  . 

"  In  hollow  pillars,  whose  ends  are  flat,  we  had  from  experi- 
ment as  before, 

Strength  in  tons  =44.3  . 

"  The  formulae  above  apply  to  all  pillars  whose  length  is  not 
less  than  about  thirty  times  the  external  diameter  ;  for  pillars 
shorter  than  this,  it  is  necessary  to  have  recourse  to  the  '  for- 
mula,' given  under  the  head  of  Strength  of  Tij^cbek,  for 


134 


CIVTL  ENGINEEEING. 


short  pillars  of  timber,  substituting  for  W  and  TP  in  that  for- 
mula, the  proper  values  applicable  to  cast-iron." 

351.  Similar  Pillars.  "  In  similar  pillars,  or  those  whose 
length  is  to  the  diameter  in  a  constant  proportion,  the  strength 
is  nearly  as  the  square  of  the  diameter,  or  of  any  other  linear 
dimension ;  or,  in  other  words,  the  strength  is  nearly  as  the 
area  of  the  transverse  section." 

"  In  hollow  pillars,  of  greater  diameter  at  one  end  than  the 
other,  or  in  the  middle  than  at  the  ends,  it  was  not  found 
that  any  additional  strength  was  obtained  over  that  of  cylin- 
drical pillars." 

"  The  strength  of  a  pillar,  in  the  form  of  the  connecting 
rod  of  a  steam-engine  "  (that  is,  the  transverse  section  pre- 
senting the  figure  of  a  cross  -f)  "was  found  to  be  very 
small,  perhaps  not  half  the  strength  that  the  same  metal 
would  have  given  if  cast  in  the  form  of  a  uniform  hollow 
cylinder." 

"  A  pillar  irregularly  fixed,  so  that  the  pressure  would  be  in 
the  direction  of  the  diagonal,  is  reduced  to  one  third  of  its 
strength.  Pillars  fixed  at  one  end  and  movable  at  the  other, 
as  in  those  fiat  at  one  end  and  rounded  at  the  other,  break  at 
one  third  the  length  from  the  movable  end ;  therefore,  to 
economize  the  metal,  they  should  be  rendered  stronger  there 
than  in  other  parts." 

352.  Long-continued  Pressure  on  Pillars.  "  To  deter- 
mine the  efiect  of  a  load  lying  constantly  on  a  pillar,  Mr. 
Fairbairn  had,  at  the  writer's  suggestion,  four  pillars  cast, 
all  of  the  same  length  and  diameter.  The  first  was  loaded 
with  4  cwt.,  the  second  with  7  cwt.,  the  third  with  10  cwt., 
and  the  fourth  with  13  cwt. ;  this  last  load  was  -^^-^  of  what 
had  previously  broken  a  pillar  of  the  same  dimensions,  when 
the  weight  was  carefully  laid  on  without  loss  of  time.  The 
pillar  loaded  with  13  cwt.  bore  the  weight  between  five  and 
six  months,  and  then  broke." 

353.  General  Properties  of  Pillars.  "In  pillars  of 
wrought-iron,  steel,  and  timber,  the  same  laws,  with  respect 
to  rounded  and  fiat  ends,  were  found  to  obtain,  as  had  been 
shown  to  exist  in  cast-iron." 

"  Of  rectangular  pillars  of  timber,  it  was  proved  experimen- 
tally that  the  pillar  of  greatest  strength  of  the  same  material, 
is  a  square." 

354.  Comparative  Strength  of  Cast-iron,  Wrou^ht- 
Iron,  Steel,  and  Timber.  "  It  resulted  from  the  experi- 
ments upon  pillars  of  the  same  dimensions  but  of  difi^erent 
materials,  that  if  we  call  the  strength  of  cast-iron  1000,  we 


STRENGTH  OF  CAST-IEON. 


135 


shall  have  for  wrought  1745,  cast  steel  2518,  Dantzic  oak 
108.8,  red  deal  78.5." 

355.  Resistance  to  Transverse  Strain.  The  following 
tables  and  deductions  are  drawn  from  the  experiments  of 
Messrs.  Hodgkinson  and  Fairbairn,  on  hot  and  cold  blast 
iron,  as  published  in  their  liejports  to  the  British  Association 
in  1837. 


Tahle  exhibiting  the  results  of  experiments  hy  Mr,  Hodg- 
kinson on  bars  of  hot  blast  iron  5  feet  long^  with  a  rect- 
angular sectional  area  j  the  bars  resting  horizontally  on 
jprojps  4  feet  6  inches  apart  j  the  weight  being  applied  at 
the  middle  of  the  bar. 


Experiment  1. 

Experiment. 

13. 

Experiment  14. 

Eectangular  bar, 
1.00  inch,  broad, 
1.00     "  deep. 
Weight  of  bar,  15  lbs.  2  oz. 

Rectangular  bar, 
1.03  inches  broad, 
3.00    "  deep. 

Rectangular  bar, 
1.02  inches  broad, 
4.98     "  deep. 
Weight  78  lbs. 

Weight  in 
lbs. 

i " 

.2  « 

o  o 

0)  £3 

<p 
fi 

<c  -a 

CQ 

.a 

+3  . 

^  m 

a 
§s 

ft 

OQ 
<D 

•s 

a 
a 

1 

a 

r 

a 

ft 

1 

o 

a 
a 

"S 

CO 

16 
23 
30 
56 
112 
224 
336 
448 
469 

.037 
.052 
.070 
.132 
.271 
.588 
.940 
1.360 
broke 

visible 
increased 
.001? 
.002 
.008 
.037 
.087 
.181 

1474 
1605 
1866 
2126 
2388 
2649 
2910 
3J72 
3433 
3694 
3956 

.130 
.156 
.185 
.212 
.243 
.272 
.307 
.340 
.378 
broke 

.001 
.003 
.006 
.010 
.012 
.017 
.023 
.030 
.038 
.050 

5867 
6798 
7730 
8661 
9593 
10524 
11087 

.127 
.153 
.177 
.207 
.235 
.275 
broke 

.01 
.03 

Ultimate  deflection 
1.444  inches. 

Ultimate  deflection 
.416inch. 

Ultimate  deflection 
.299  inch. 

356.  The  following  remarks  are  extracted  from  the  same 
Report :  "  I  had  remarked,  in  some  of  the  experiments,  that 
the  elasticity  of  the  bars  was  injured  much  earlier  than  is 
generally  conceived ;  and  that  instead  of  its  remaining  per- 
lect  till  one  third,  or  upwards,  of  the  breaking  weight  was 
laid  on,  as  is  generally  admitted  by  writers,  it  was  evident 
that  fth,  or  less,  produced  in  some  cases  a  considerable  set  or 
defect  of  elasticity ;  and  judging  from  its  slow  increase  after- 


136 


CIVIL  ENGINEERING. 


Hesults  of  experiments^  hy  the  same,  on  the  Transverse 
Strength  of  Cold  Blast  Iron  ;  length  of  bars,  and  distance 
between  tlve  jpohits  of  sujyport  the  same  as  in  the  preced- 
ing table. 


EXPEEIMENT  1. 


Rectangular  bar, 
1.025  inch  deep, 
1.0U2    "  broad. 
Weight,  15  lbs.  6  oz. 


d 

.a 

Weight! 
lbs. 

eflection 
inches. 

Set  in 
inches. 

« 

16 

.033 

visible 

30 

.062 

increased 

56 

.120 

.002 

112 

.240 

.007 

168 

.370 

.014 

224 

.510 

.028 

280 

.649 

.041 

836 

.798 

.061 

892 

.953 

.084 

448 

1.120 

.120 

504 

1.310 

.170 

514 

it  bore 

618 

broke 

Ultimate  deflection 
1.3tt  inch. 


EXPEBIMENT  1?. 


Rectangular  bar, 
3.00  inches  deep, 
1.02     "  broad. 
Weight,  46  lbs.  8  oz. 


1082 
1343 
1605 
1866 
2126 
2388 
2649 
2910 
3172 
3433 
3694 
3825 


.091 
.111 
.138 
.164 
.190 
.229 
.250 
.281 
.310 
.345 
.378 
broke 


.003 
.006 
.008 
.010 
.012 
.015 
.019 
.026 
.031 
.037 
.046 


Ultimate  deflection 
0.395  inch. 


EXPEBIMENT  13. 


Rectangular  bar, 
4.98  inches  deep, 
1.03     "  broad. 
Weight,  7b  lbs. 


Weight  in 
lbs. 

Deflection  in 
inches. 

!• 

Sot  in 
inches. 

4936 

.110 

.013 

5867 

.130 

6798 

.153 

.020 

7730 

.179 

.025 

8662 

.195 

9593 

.219 

.034 

10525 

.250 

.042 

10588 

broke 

Ultimate  deflection 
0.252. 


wards,  I  was  persuaded  that  it  had  not  come  on  by  a  sudden 
change,  but  had  existed,  though  in  a  less  degree,  from  a  very 
early  period." 

"  From  what  has  been  stated  above,  deduced  from  experi- 
ments made  with  great  care,  it  is  evident  that  the  maxim  of 
loading  bodies  within  the  elastic  limit  has  no  foundation  in 
nature  ;  but  it  will  be  considered  as  a  compensating  fact, 
that  materials  will  bear  for  an  indefinite  period  a  much 
greater  load  than  has  hitherto  been  conceived." 

357.  "  We  may  admit,"  from  the  mean  results,  "  that  the 
strength  of  rectangular  bars  is  as  the  square  of  the  depth." 

358.  EiFects  of  Time  upon  the  Deflections  caused  by  a 
Permanent  Load  on  the  Middle  of  Horizontal  Bars.  Tlie 
following  table  exhibits  the  I'esults  of  Mr.  Fairbairn's  experi- 
ments  on   this   point.    The  experiments  were  made  on 


STRENGTH  OF  CAST-IEON. 


137 


bars  5  feet  long,  1.05  inch  deep  ;  the  one  of  cold  blast  iron, 
1.03  inch  broad ;  the  other  of  hot  blast,  1.01  broad  ;  distance 
between  the  points  of  support  4  feet  6  inches.  The  constant 
weight  suspended  at  the  centre  of  the  bars  was  280  lbs.  This 
weight  remained  on  from  March  11th,  1837,  to  June  23d, 
1838. 


Cold  blast  iron. 
Deflection  in 
inclies. 

Date  of  observation. 

Temp. 

Hot  blast  iron. 
Deflection  in 
inches. 

Eatio  of  increase  of 
deflections. 

.930 
.963 

March  11th,  1837, 
June  33d,  1838. 

78" 

1.064 
1.107 

.033 

Increase, 

.043 

1000  :  1303 

359.  Mr.  Fairbairn  in  his  Report  remarks  on  the  above 
and  like  results :  "  The  hot  blast  bar  in  these  experiments 
being  more  deflected  than  the  cold  blast,  indicates  that  the 
particles  are  more  extended  and  compressed  in  the  former 
iron,  with  the  same  weight,  than  in  the  latter.  This  excess 
of  deflection  may  in  some  degree  account  for  the  rapidity  of 
increase,  which  it  will  be  observed  is  considerably  greater  in 
the  hot  than  in  the  cold  blast  bar." 

"  It  appears  from  the  present  state  of  the  bars  (which  indi- 
cate a  slow  but  progressive  increase  in  the  deflections)  that 
we  must  at  some  period  arrive  at  a  point  beyond  their  bearing 
powers  ;  or  otherwise  to  that  position  which  indicates  a  cor- 
rect adjustment  of  the  particles  in  equilibrium  with  the  load. 
Which  of  the  two  points  we  have  in  this  instance  attained  is 
diflicult  to  determine  ;  sufficient  data,  however,  are  adduced  to 
show  that  the  weights  are  considerably  beyond  the  elastic 
limit,  and  that  cast  iron  will  support  loads  to  an  extent  be- 
yond what  has  usually  been  considered  safe,  or  beyond  that 
point  where  a  permanent  set  takes  place." 

360.  Effects  of  Temperature.  Mr.  Fairbairn  remarks : 
"  The  infusion  of  heat  into  a  metallic  substance  may  render  it 
more  ductile,  and  probably  less  rigid  in  its  nature  ;  and  I  ap- 
prehend it  will  be  found  weaker,  and  less  secure  under  the 
effects  of  heavy  strain.  This  is  observable  to  a  considerable 
extent  in  the  experiments  "  on  transverse  strength  "  ranging 
from  26°  up  to  190°  Fahr." 

"The  cold  blast  at  26°  and  190°,  is  in  strength  as  874  :  743. 
The  hot  blast  at  26°  and  190°,  is  in  strength  as  811  :  731. 


138 


CIVIL  ENGrNEEEING. 


Being  a  diminution  in  strength  as  100  :  85  for  the  cold  blast, 
and  100  to  90  for  the  hot  blast,  or  15  per  cent,  loss  of  strength 
in  the  cold  blast,  and  ten  per  cent,  in  the  hot  blast." 

"  A  number  of  the  experiments  made  on  Xo.  3  iron  have 
given  extraordinary  and  not  unf  requently  unexpected  results. 
Generally  speaking,  it  is  an  iron  of  an  irregular  character, 
and  presents  less  uniformity  in  its  texture  than  either  the  first 
or  second  qualities ;  in  other  respects  it  is  more  retentive,  and 
is  often  used  for  giving  strength  and  tenacity  to  the  finer 
metals." 

"  At  212°  we  have  in  the  'No.  3  a  much  greater  weight  sus- 
tained than  what  is  indicated  by  the  No.  2  at  190°  ;  and  at 
600°  there  appears  in  both  hot  and  cold  blast  the  anomaly  of 
increased  strength  as  the  temperature  is  advanced  from  boil- 
ing water  to  melted  lead,  arising  from  the  greater  strength  of 
the  No.  3  iron." 

361.  From  experiments  made  by  Major  Wade  on  American 
cast  iron,  and  by  Mr.  Fairbairn  on  English  cast  iron,  it  appears 
that  the  tenacity  of  the  metal  is  increased  both  by  remelting, 
and  by  prolonged  fusion  when  kept  in  their  certain  limits. 
It  also  appears  from  other  experiments  that  repeated  fusions 
occasion  a  heavy  waste  of  material,  and  that  if  either  remelt- 
ing or  prolonged  fusion  be  carried  too  far  the  result  will  be 
an  iron  of  a  hard  and  brittle  quality. 

362.  Influence  of  Form  upon  the  Transverse  Strength 
of  Cast  Iron  Beams.  Upon  no  point,  respecting  the  strength 
of  cast  iron,  have  the  experiments  of  Mr.  Hodgkinson  led  to 
more  valuable  results  to  the  engineer  and  architect,  than  upon 
the  one  under  this  head.  The  following  tables  give  the  results 
of  experiments  on  bars  of  a  uniform  cross-section  (thus  T) 
cast  from  hot  and  cold  blast  iron.  The  bars  were  7  feet 
long,  and  placed,  for  breaking,  on  supports  6  feet  6  inches 
asunder. 


STEENGTH  OF  CAST-IEON. 


139 


Table  exhibiting  the  Results  of  Experiments  on  bars  of  Hot 
Blast  Iron  of  the  form  of  cross  section  as  above. 


EXPEKIMENT  4. 

Bar  broken         Q  as  shown 

with  the  rib  downward. 


Weight  in  lbs. 

Deflection  in 
inches. 

Set. 

Weight  in  lbs. 

Deflection  in 
inches. 

Set. 

7 

.015 

visible 

7 

not  visible 

14 

.032 

.001 

14 

.025 

visible 

21 

.046 

.002 

21 

.045 

.002 

28 

.064 

.004 

28 

.065 

.003 

56 

.130 

.005 

56 

.134 

.005 

112 

.273 

.020 

112 

.270 

.015 

168 

.444 

.035 

224 

.580 

.058 

224 

.618 

.058 

336 

.895 

.101 

280 

.813 

.093 

448 

1.224 

.155 

336 

1.030 

.130 

560 

1.585 

.235 

364 

broke 

672 

1.985 

.330 

784 

2.410 

.490 

896 

3.450 

.723 

1008 

4.140 

1.040 

1064 

1120 

broke 

Ultimate  deflection  1.138  inches. 

Fracture  caused  by  a  wedge  2.92  inches 
long  and  1.05  deep,  of     ^-A,.^  this 
form  flying  out.  /"""^ 

Ultimate  deflection  4.830. 

EXPEBIMENT  5. 


Bar  broken         B  as  shown 

with  the  rib  upward. 


N'ote.  The  annexed  diagram  shows  the 
form  of  the  miiform  cross-section  of  the 
bars.  The  linear  dimensions  of  the  cross- 
section  in  the  two  experiments  were  as  fol- 
lows : — 


Length  of  parallelogram  AB 
Depth  "  AB 

Total  depth  of  bar  CD 

Breadth  of  rib   DE 


5  inches 
0.30  " 
1.55  " 
0.36  " 


Expt.  4. 


5  inches  ^ 
0.30  "  1 
1.56  "  f 
0.365  "  J 


Expt.  5. 


140 


CIVIL  ENGESTEEEING. 


Tahle  exhibiting  Results  of  Experiments  on  hars  of  Cold 
Blast  Iron  5  feet  long^  of  the  same  form  of  cross  section 
as  in  preceding  table. 


EXPEBJMENT  4. 


Bar  broken        ■  with  rib 

downward. 


Ultimate  deflection  36. 


W Gig^ub  in  Ids. 

Deflection  in 
inches. 

Set. 

Wciglit  in  lbs. 

Deflection  in 
inches. 

oeu. 

112 

.03 

112 

.03 

224 

.07 

224 

.07 

336 

.11 

336 

.11 

392 

.13 

.005 

448 

.15 

420 

.14 

.007 

560 

.19 

.005 

448 

.15 

.010 

616 

.21 

.010 

560 

.19 

.012 

672 

.23 

672 

.23 

.015 

728 

.015 

784 

.28 

.023 

784 

.27 

896 

.33 

.030 

896 

.31 

952 

.35 

1008 

.35 

980 

broke 

1120 

.39 

1344 

.48 

1568 

.57 

1792 

.67 

2016 

.80 

2240 

.95 

2296 

it  bore 

2352 

broke 

EXPEEIMENI  5. 


Bar  broken         I  with  nb 

upward. 


Ultimate  deflection  1.03. 


Fracture  by  a  wedge  breaking 
Experiment  5,  Hot  Blast. 


Ifote.  The  linear  dimensions  of  the  cross-section  of  the  bars 
in  the  above  table  were  nearly  the  same  as  those  in  the  pre- 
ceding table,  with  the  exception  of  the  total  depth  CD,  which 
in  these  last  two  experiments  was  2.27  inches,  or  a  little 
more. 

363.  The  object  had  in  view  by  Mr.  Ilodgkinson,  in  the 
experiments  recorded  in  the  two  preceding  tallies,  was  two- 
fold ;  the  one  to  ascertain  the  circumstances  under  which  a 
permanent  set,  or  injury  to  elasticity  takes  place ;  the  other 
to  ascertain  the  effect  of  tlie  form  of  cross  section  on  the 
transverse  strength  of  cast  iron.  Tlie  following  extracts  from 
the  Report,  give  the  principal  deductions  of  Mr.  Ilodgkinsoii 
on  these  points. 

"  In  experiments  4  and  5  "  (on  hot  blast  iron),  "  which  were 


STEENGTH  OF  CAST-IEON. 


141 


on  longer  bars  than  the  others,  cast  for  this  purpose,  and  for 
another  mentioned  further  on,  the  elasticity  (in  Expt.  4)  was 
sensibly  injured  with  Y  lbs.,  and  in  the  latter  (Expt.  5)  with 
14  lbs.,  the  breaking  weights  being  364  lbs.,  and  1120  lbs. 
In  the  former  of  these  cases  a  set  was  visible  with  -g^,  and  in 
the  other  with  -J-q-  of  the  breaking  weight,  showing  that  there 
is  no  weight,  however  small,  that  will  not  injure  the  elasti- 
city." 

"  AYhen  a  body  is  subjected  to  a  transverse  strain,  some  of 
its  particles  are  extended  and  others  compressed ;  I  was  de- 
sirous to  ascertain  whether  the  above  defect  in  elasticity  arose 
from  tension  or  compression,  or  both.  Experiments  4  and  5 
show  this ;  in  these  a  section  of  the  casting,  which  was  uni- 

e 

form  throughout,  had  the  form  i.    During  the  experiments 

a  b 

the  broad  part  ah  was  laid  horizontally  upon  supports  ;  the 
vertical  rib  c  in  the  latter  experiment  being  upward,  in  the 
former  downward.  When  it  was  downward  the  rib  was  ex- 
tended, when  upward  the  rib  was  compressed.  In  both  cases 
the  part  ah  was  the  fulcrum  ;  it  w^as  thin,  and  therefore  easily 
flexible;  but  its  breadth  was  such  that  it  was  nearly  inex- 
tensible  and  incompressible,  comparatively,  with  the  vertical 
rib.  We  may  therefore  assume,  that  nearly  the  whole  flexure 
which  takes  place  in  a  bar  of  this  form,  arises  from  the  ex- 
tension or  compression  of  the  rib,  according  as  it  is  downward 
or  upward.  In  Expt.  4  we  have  extension  nearly  without 
compression,  and  in  Expt.  5  compression  almost  without  ex- 
tension. These  experiments  were  made  with  great  care. 
They  show  that  there  is  but  little  difference  in  the  quantity 
of  set,  whether  it  arises  from  tension  or  compression." 

"  The  set  from  compression,  however,  is  usually  less  than 
that  from  extension,  as  is  seen  in  the  commencement  of 
the  two  experiments,  and  near  the  time  of  fracture  in  that 
submitted  to  tension.  The  deflections  from  equal  weights 
are  nearly  the  same  whether  the  rib  be  extended  or  compress- 
ed, but  the  ultimate  strengths,  as  appears  from  above,  are 
widely  different." 

364.  Form  of  Cast  Iron  Beam  best  adapted  to  Resist  a 
Transverse  Strain.  The  experiments  of  Mr.  Hodgkinson 
on  this  subject,  pub-ished  in  thelfemoirs  of  the  Literary  and 
Philosojyhical  Society  of  Manchester^  Second  Series,  vol.  5, 
are  of  equal  interest  with  those  just  detailed,  both  in  their 
general  results  and  practical  bearing.  From  these  experi- 
ments, the  conclusion  drawn  is  that  the  form  of  beam  in  the 


142 


CIYIL  ENGINEEErCTG. 


annexed  diagrams  is  the  most  favorable  for  resistance  to 
transverse  strains. 


Fig.  a. 


Fig.  h. 


Fig.  e. 


Fig.  a  represents  the  plan,  Fig.  h 
the  elevation,  and  Fig.  c  the  cross 
section  (enlarged)  at  the  middle  of 
the  beam.  From  the  Figs,  it  will 
be  seen  that  the  beam  consists  of 
three  parts  ;  a  bottom  flanch  of  uni- 
form depth,  but  variable  breadth, 
tapering  from  the  centre  towards 
the  extremities,  where  the  points 
of  support  would  be  placed  so  as  to 
form  a  portion  of  the  common  parabola  on  each  side  of  the 
axis  of  the  beam,  the  vertex  of  each  parabola  being  at  the 
centre  of  the  beam.  The  object  of  this  form  of  flanch  was  to 
make  it,  according  to  theory,  the  strongest,  with  the  same 
amount  of  material,  to  bear  a  weight  uniformly  distributed 
over  it.  The  top  flanch  is  of  a  like  form,  but  of  much  small- 
er breadth  and  depth  than  the  bottom  one.  The  two  are 
united  by  a  vertical  rib  of  uniform  depth  and  breadth. 

The  following  are  the  relative  dimensions  of  this  form  of 
beam,  which,  from  experiment,  gave  the  most  favorable 
result. 

Distance  of  supports  4  ft.  6  inches. 

Total  depth  of  beam  0  "  5i  " 

Breadth  of  top  flanch  at  centre  of  beam   2.33  " 

"  bottom  flanch    6.66  " 

Uniform  depth  of  top  flanch   0.31  " 

"  bottom  flanch   0.06  " 

Thickness  of  vertical  rib   0.266  " 

Total  area  of  cross  section   6.4  square  inch. 

Weight  of  beam  71  lbs. 

"This  beam  broke  in  the  middle  by  compression  with 
260S4  lbs.,  or  11  tons  13  cwt.,  a  wedge  separating  from  its 
upper  side." 


STRENGTH  OF  CAST-IEON. 


143 


"  The  weights  were  laid  gradually  and  slowly  on,  and  the 
beam  had  borne  within  a  little  of  its  breaking  weight  a  con- 
siderable time,  perhaps  half  an  hour." 

"  The  form  of  tlie  fracture  and  wedge  is  represented  in  the 
Fig.  5,  where  enfi^  the  wedge,  ^=5.1  inches,  ^?i=3.9  inches, 
angle  6/?/=  82°.'' 

"  It  is  extremely  probable,  from  this  fracture,  that  the  neu- 
tral point  was  at  the  vertex  of  the  wedge,  and  therefore  at 
fths  the  depth  of  the  beam,  since  3.9 =f  x  5-J  nearly." 

The  relative  dimensions  above  given  were  arrived  at  by 
"  constantly  making  small  additions "  to  the  bottom  flanch, 
until  a  point  was  reached  where  resistance  to  compression 
could  no  longer  be  sustained.  The  beams  of  this  form,  in  all 
previous  experiments,  having  yielded  by  the  bottom  flanch 
tearing  asunder. 

"  The  great  strength  of  this  form  of  cross  section  is  an  in- 
disputable refutation  of  that  theory  which  would  make  the 
top  and  bottom  ribs  of  a  cast  iron  beam  equal." 

"  The  form  of  cross  section  "  (as  above)  "  is  the  best  which 
we  have  arrived  at  for  the  beam  to  bear  an  ultimate  strain. 
If  we  adopt  the  form  of  beam  (as  above)  I  think  we  may 
confidently  expect  to  obtain  the  same  strength  with  a  saving 
of  upwards  of  ^th  of  the  metal." 

365.  Rules  for  determining  the  Ultimate  Strength  of  Cast 
Iron  Beams  of  the  above  Forms,  From  the  results  of  his  ex- 
periments, Mr.  Hodgkinson  has  deduced  the  following  very 
simple  formulae,  for  determining  the  breaking  weight,  in  tons, 
when  applied  at  the  middle  of  a  beam. 

Call  the  breakino;  weio;ht  in  tons,  W. 

Call  the  area  of  the  cross  section  of  the  bottom  flanch,  taken 
at  the  middle  of  the  beam,  a. 

Call  the  depth  of  the  beam  at  the  middle  point,  d. 
Call  the  distance  between  the  supports,  I. 
Then 

I 

when  the  beam  has  been  cast  with  the  bottom  flanch  upward 
and  * 

Tr=24^, 

when  the  beam  has  been  cast  on  its  side. 

The  working  strain  on  cast  iron  beams  subjected  to  direct 
compression  is  placed  by  most  authorities  at  from  -J-th  to  -^th 
of  the  crushing  weight,  when  the  beam,  a  column  for  exam- 


144 


CIVIL  ENGINEERmG. 


pie,  is  not  subjected  to  violent  vibrations  or  shocks.  In  the 
contrary  case,  particularly  in  beams  subjected  to  a  transverse 
strain,  it  is  recommended  to  reduce  the  working  strain  to  yV^^ 
the  crushing  strain. 

366.  Effect  of  Horizontal  Impact  upon  Cast  Iron  Bars. 
The  following  tables  of  experiments  on  this  subject,  and  the 
results  drawn  from  them,  are  taken  from  a  paper  by  Mr. 
Hodgkinson,  published  in  the  Fifth  Rejport  of  the  British 
Association. 

The  bars  under  experiment  were  impinged  upon  by  a 
weight  suspended  freely  in  such  a  position  that,  hanging  ver- 
tically, it  was  in  contact  with  the  side  of  the  bar.  The  blow 
was  given  by  allowing  the  weight  to  swing  through  different 
arcs.  The  bars  were  so  confined  against  lateral  supports,  that 
they  could  take  no  vertical  motion. 

Table  of  experiments  on  a  cast  iron  har,  ^ft.  6  in.  long,  1  in. 
hroaa,  -J  in.  thicJc,  weighing  7i  lbs.,  placed  with  the  broad- 
side against  lateral  supports  4  ft.  asunder,  and  impinged 
iipon  by  cast  iron  and  lead  balls  weighiny  8^  lbs.,  swinging 
through  arcs  of  the  radius  Vlfeet. 


Impact  with  leaden  ball. 

Impact  ■with  iron  ball. 

Chord    of  arc 
fallen  through 
in  feet. 

Observed  chord 
of  recoil  of  ball 
in  inches. 

1 

Observed  deflec- 
tion of  bar  in 
inches. 

Chord    of  arc 
fallen  through 
in  feet. 

Observed  chord 
of  recoil  of  ball 
in  inches. 

1 

Observed  deflec- 
tion of  bar  in 
inches. 

1 

2 
3 
4 
5 
6 

6.5 

13 
19 
27 
34 
47 

.24 
.46 
.73 
.97 
1.30 
1.60 

1 

2 
3 
4 
5 
6 

6.5 
14 
20 
29 
37 
48 

.23 

.46 
.65 
.98 
1.32 
1.65 

"  Before  the  experiments  on  impact  were  made  upon  this 
bar,  it  was  laid  on  two  horizontal  supports  4  feet  a^mder,  and 
weights  gently  laid  on  the  middle  bent  it  (in  the  same  direc- 
tion that  it  was  afterwards  bent  by  impact)  as  below ; 

28  lbs.  beat  it  .37  inch. 

56  lbs.    "       .77  inch.    Elasticity  a  little  injured." 


STRENGTH  OF  CAST-IEON. 


145 


TcMe  of  experiments  on  a  cast  iron  har  7  ft.  long,  1.08  in. 
hroad  and  1.05  in.  tkich,  weighing  23J-  Ws..,  placed,  as  in 

preceding  experiments,  against  supports  6  ft.  6  in. 
asimder,.  and  bent  hy  impacts  in  the  middle.  Impinging 
hall  of  cast  iron  weighing  2 Of  lbs.    Radius  of  arcs  16 

feet. 


Impact  upon  bar. 

Impact  upon  the 
weight. 

Chord     of  arc 
fallen  through. 

Observed  deflec- 
tion in  inches. 

Chord    of  arc 
fallen  through. 

Observed  deflec- 
tion in  inches. 

2 
3 
4 
5 
6 
7 
8 

.46 
.62 

.87 
1.03 
1.24 
1.44 
1.80 

2 
3 
4 
5 
6 
7 
8 
9 

.31 
.43 
.69 
.81 
1.04 
1.28 
1.41 
1.63 

The  results  in  the  3d  and  4th  columns  of  the  above  table 
were  derived  from  allowing  the  ball  to  impinge  against  a 
weight  of  56  lbs.,  hung  so  as  to  be  in  contact  with  the  bar. 

"  Before  the  experiments  on  impact,  the  beam  was  laid  on 
two  supports  6  ft.  6  in.  asunder,  and  was  bent  .78  in.  by  123- 
lbs.  (including  the  pressure  from  its  own  weight),  applied' 
gently  in  the  middle." 

Tables  of  experiments  on  two  cast  iron  bars,  4  ft.  6  in.  long,..  . 
full  inch  square,  weighing  14  lbs.  10  ob.  nearly,  placed 
against  supports  4:  feet  apart,  and  impinged  uponby  a  cast 
iron  bcdl  weighing  44  lbs.    Badius  l^ft. 


Impact  in  the  middle. 

Impact  at  one-fourth  the  length  froni  the  middle 
of  the  bars. 

Chords  of  arcs  in 
feet. 

Mean  deflections 
of  the  two  bars 
in  inches. 

Chords  of  arcs  in 
feet. 

Mean  deflections 
of  the  two  bars 
in  inches. 

Mfean  ratio  of  the 
deflections  in 
the  two  cases. 

2 

3 
4 
5 

5.5 
6 

.35 
.55 
.77 
.95 
1.05 
Broke  in  the 
middle 

2 
3 
4 
5 

5.5 
6 

.24 
.42 
.52 
.64 
.70 

Brol  e  at  th^ 
point  of  impa  t 

694 

10 


146 


CIVIL  EXGIXEERING. 


The  results  in  the  1st  of  the  above  tables  are  from  bars 
struck  in  the  middle,  those  in  the  2d  table  are  from  bars 
struck  at  the  middle  ]3oint  between  the  centre  and  extremity 
of  the  bar. 

From  the  al)()ve  and  other  experiments  the  conclusion  is 
drawn,  "  that  a  uniform  beam  will  bear  the  same  blow,  whether 
struck  in  the  middle  or  half  way  between  that  and  one  end." 

"  From  all  the  experiments  it  appears  that  the  deflection  is 
nearly  as  the  chord  of  the  arc  fallen  through,  or  as  the  velo- 
city of  impact." 

The  following  conclusions  are  drawn  from  the  experiments. 

(1.)  "  If  different  bodies  of  equal  weight,  but  differing  con- 
siderably in  hardness  and  elastic  force,  be  made  to  strike  hori- 
zontally against  the  middle  of  a  heavy  beam  supported  at  its 
ends,  all  the  bodies  will  recoil  with  velocities  ecpial  to  one 
another." 

(2.)  "  If,  as  before,  a  beam  supported  at  its  ends  be  struck 
horizontally  by  bodies  of  the  same  weight,  but  different  hard- 
ness and  elastic  force,  the  deflection  oi  the  beam  will  be  the 
same  whichever  body  be  used." 

(3.)  "  The  quantity  of  recoil  in  a  body,  after  striking 
against  a  beam  as  above,  is  nearly  equal  to  (though  somewhat 
below)  what  would  arise  from  the  full  varying  pressure  of  a 
perfectly  elastic  beam,  as  it  recovered  its  form  after  deflec- 
tion." 

Note.  This  last  conclusion  is  drawn  from  a  comparison  of 
the  results  of  experiment  with  those  obtained  from  calcula- 
tion, in  which  the  beam  is  assumed  as  perfectly  elastic. 

(4.)  "  The  effect  of  bodies 'of  different  natures  striking 
against  a  hard,  flexible  beam,  seems  to  be  independent  of  the 
elasticities  of  the  bodies,  and  may  be  calculated,  with  trifling 
error,  on  a  supposition  that  they  are  inelastic." 

(5.)  "The  power  of  a  uniform  beam  to  resist  a  blow  given 
horizontally,  is  the  same  in  whatever  part  it  is  struck." 

367.  From  the  results  of  the  experiments  of  ^Messrs.  Fair- 
bairn  and  Ilodgkhison,  on  'the  properties  of  cold  and  liot  blast 
iron,  it  appears  that  the  ratio  of  their  resistances  to  impact  is 
1000  to  1226.3,  the  resistance  of  cold  blast  being  represented 
by  1000  :  the  resistance,  or  power  of  the  beam  to  bear  a  liori- 
zontal  impact,  being  measured  by  the  product  of  its  breaking 
weight  from  a  transverse  strain  at  the  middle  of  the  beam 
and  its  ultimate  deflection.  This  measure,  Mr.  Hodgkinson 
remarks,  "supposes  that  all  cast  iron  bars  of  the  same  dimen- 
sions, in  our  experiments,  are  of  the  same  weight,  and  that 
the  deflection  of  a  beam  up  to  the  breaking  weight  would  be 


STRENGTH  OF  WROUGHT  IRON.  147 

as  tlie  pressure.  ITeither  of  these  is  true ;  they  are  only 
approximations  ;  but  the  difference  in  the  weights  of  cast  iron 
bars  of  equal  size  is  very  little,  and,  taking  them  as  the  same, 
it  may  be  inferred  from  my  paper  on  Impact  upon  Beams 
{Fifth  Rejport  of  the  British  Association)  that  the  assump- 
tion above  gives  results  near  enough  for  practice." 


YI. 

STRENGTH  OF  WROUGHT  IRON. 

This  material,  from  its  very  extensive  applications  in 
structures  where  a  considerable  tensile  force  is  to  be  resisted, 
as  in  suspension  bridges,  iron  ties,  etc.,  has  been  the  subject 
of  a  very  great  number  of  experiments.  Among  the  many 
may  be  cited  those  of  Telford  and  Brown  in  England,  Duleau 
in  France,  and  the  able  and  extensive  series  upon  plate  iron 
for  steam  boilers,  made  under  the  direction  of  the  Franklin 
Institute,  and  published  in  the  19th  and  20th  vols.  {B'ew 
Series)  of  the  Joui^nal  of  the  Institute. 

368.  Resistance  to  Tensile  Strain.  The  tables  on  the 
next  page  exhibit  the  tensile  strength  of  this  material  under 
ordinary  temperatures,  and  in  the  different  states  in  which  it 
is  used  for  structures. 

It  is  remarked,  in  the  Eeport  of  the  Sub-committee,  "that 
the  inherent  irregularities  of  the  metal,  even  in  the  best  speci- 
mens, whether  of  rolled  or  hammered  iron,  seldom  fall  short 
of  10  or  15  per  cent,  of  the  mean  strength." 

From  the  same  series  of  experiments,  it  appears  that  the 
strength  of  rolled  plate  lenghthwise  is  about  6  per  cent, 
greater  than  its  strength  crosswise. 

In  the  Tenth  Report  of  the  British  Association  in  1840, 
Mr.  Fairbairn  has  given  the  results  of  experiments  on  plate 
iron  by  Mr  Ilodgkinson,  from  which  it  ap]3ears  that  the  mean 
strength  of  iron  plates  lengthwise  is  22.52  tons. 

Crosswise  "  23.04  " 
Single-riveted  plates     "  18,590  lbs. 
Double-riveted  plates  "  22,258  " 

Representing  the  strength  of  the  plate  by  100. 

The  double-riveted  plates  will  be   70. 

The  single-riveted  plates  will  be   56. 


148  CIVIL  EXGINEEEING. 


Table  exhihiting  the  Strength  of  Square  and  Round  Bars  of 
Wrought  Iron. 


DESCRIPTION  OF  IRON. 

Length  of 
pieces  in 
feet. 

Extension  be- 
fore rupture 
in  inches. 

Breaking 
weight  in 
tons. 

Tensile 
strength  per 
square  inch. 

Author. 

1 

22.75 

29 

29 

Telford. 

1 

0.375 

29 

29 

1 

2.2 

100 

29.28 

3..5 

0.19 

40.95 

2;i75 

Brown. 

"    1.19          "  "   

3.00 

33.50 

23.75 

Round  bar,  1.31  in.  diam.,  Russian 

3.5 

2.25 

36.10 

26.50 

3.5 

2.00 

38.05 

24.35 

Bars  reduced  in  the  middle  by 

12.5 

18.50 

82.75 

26.;3;3 

hammering  to  0.375  in.  square 
0.50 

31.35 

Brunei. 

30.80 

21.38 

j  Frankhn 
{ Institute. 

22.32 

2:3.25 

"          Salisbury,  Connecticut.. 

25.89 

25.97 

(I 

26.07 

"          Lancaster  Co..,  Penn  

26.18 

"   (cable  iron)  English 
"      do.  hammer-hardened  " 

26.62 

31.70 

"  Russian 

as. 95 

(I 

Wire,  0.333  in.  diam.  Phillipsburg 
"  0.190 

37.58 

(( 

32.98 

"      0.156      "  " 

39.80 

»      0.10        "  English 

35.81 

Telford. 

Table  exhihiting  the  Mean  Strength  of  Boiler  Iron,  jper 
square  inch  in  lbs.,  cut  from  plates  with  shears. 


Process  of  manufacture. 


Piled  iron  

Hammered  plate 
Puddled  iron. . .  . 


Rough  edge  bar. 


58,045 
47,506 
52,341 


Edges    filed  uni- 
formly. 


56,081 
55,584 
51,039 


Notches  filed  into 
bar  on  each  edge. 


63,266 
58.447 
62,420 


Professor  Barlow,  in  his  Report  to  the  Directors  of  the 
London  and-  Birmingham  Railroad  (Journal  of  Franklin 
Institute,  July,  1835),  states,  as  the  results  of  his  experiments, 
that  a  bar  of  malleable  iron  one  inch  square  is  elongated  the 

-i-Q^th  part  of  its  length  by  a  strain  of  one  ton  ;  that  good 
iron  is  elongated  the  j-J^th  part  by  a  strain  of  10  tons,  and 
is  injured  by  this  strain,  while  indifferent  or  bad  iron  is  in- 
jured by  a  strain  of  8  tons. 

From  the  Report  made  to  the  Franklin  Institute,  it  appears 
that  the  first  set,  or  permanent  elongation,  may  take  place 
under  very  different  strains,  varying  with  the  character  of  the 
material.    The  most  ductile  iron  yields  permanently  to  a  low 


STRENGTH  OF  WEOUGHT  lEON. 


149 


degree  of  strain.  The  extremes  by  which  a  permanent  set  is 
given  vary  between  the  0.416  and  0.872  of  the  ultimate 
strength  ;  the  mean  of  thirteen  comparisons  being  0.641. 

From  the  able  series  of  experiments  made  by  Mr.  Kirkaldy 
at  Glasgow,  on  the  tensile  strength  of  wrought  iron,  he  has 
arrived  at  the  following  general  conclusions  {Kirkaldy^ 
Experiments  on  Wrought  Iron  and  Steel,  2d  Ed.,  1866) : — 

1.  The  breaking  strain  does  not  indicate  the  quality,  as 
hitherto  assumed. 

2.  A  high  breaking  strain  may  be  due  to  the  iron  being  of 
superior  quality,  dense,  fine,  and  moderately  soft,  or  simply 
to  its  being  very  hard  and  unyielding. 

3.  A  low  breaking  strain  may  be  due  to  looseness  and 
coarseness  in  the  texture,  or  to  extreme  softness,  although 
very  close  and  fine  in  quality. 

4.  The  contraction  of  area  at  fracture,  previously  overlook- 
ed, forms  an  essential  element  in  estimating  the  quality  of 
specimens. 

5.  The  respective  merits  of  various  specimens  can  be  cor- 
rectly ascertained  by  comparing  the  breaking  strain  jointly 
with  the  contraction  of  area. 

6.  Inferior  qualities  show  a  much  greater  variation  in  the 
breaking  strain  than  superior. 

7.  Greater  differences  exist  between  small  and  large  bars 
in  coarse  than  in  fine  varieties. 

8.  The  prevailing  opinion  of  a  rough  bar  being  stronger 
than  a  turned  one  is  erroneous. 

9.  Rolled  bars  are  slightly  hardened  by  being  forged 
down. 

10.  The  breaking  strain  and  contraction  of  area  of  iron 
plates  are  greater  in  the  direction  in  which  they  are  rolled 
than  in  a  transverse  direction. 

11.  A  very  slight  difference  exists  between  specimens  froiu 
the  centre  and  specimens  from  the  outside  of  crank-shafts. 

12.  The  breaking  strain  and  contraction  of  area  are  greater 
in  those  specimens  cut  lengthways  out  of  crank-shafts  than  in 
those  cut  crossways. 

13.  Iron,  when  fractured  suddenly,  presents  invariably  a 
crystalline  appearance ;  when  fractured  slowly,  its  appearance 
is  invariably  fibrous. 

14.  The  appearance  may  be  changed  from  fibrous  to  crys- 
talline by  merely  altering  the  shape  of  specimen  so  as  to 
render  it  more  liable  to  snap. 

15.  The  appearance  may  be  changed  by  varying  the  treat- 
ment so  as  to  render  the  iron  harder  and  more  liable  to  snap. 


150 


CIVIL  ENGINEERING. 


16.  The  appearance  may  be  changed  by  applying  the 
strain  so  suddenly  as  to  render  the  specimen  more  liable  to 
snap,  from  having  less  time  to  stretch. 

17.  Iron  is  less  liable  to  snap  the  more  it  is  worked  and 
rolled. 

18.  The  "skin,"  or  outer  part  of  the  iron,  is  somewhat 
harder  than  the  inner  part,  as  shown  by  appearance  of  frac- 
ture in  rough  and  turned  bars. 

19.  The  mixed  character  of  the  scrap-iron  used  in  large 
forgings  is  proved  by  the  singularly  varied  appearance  of  the 
fractures  of  specimens  cut  out  of  crank-shafts. 

20.  The  texture  of  various  kinds  of  wrought  iron  is  beauti- 
fully developed  by  immersion  in  dilute  hydrochloric  acid, 
which,  acting  on  the  surrounding  impurities,  exposes  the 
metallic  portion  alone  for  examination. 

21.  In  the  fibrous  fractures  the  threads  are  drawn  out,  and 
are  viewed  externally,  whilst  in  the  crystalline  fractures  the 
threads  are  snapped  across  in  clusters,  and  are  viewed  inter- 
nally or  sectionally.  In  the  latter  cases  the  fracture  of  -the 
specimen  is  always  at  right  angles  to  the  length ;  in  the 
former  it  is  more  or  less  irregular ;  fracture  is  nearly  free 
of  lustre  and  unlike  the  crystalline  appearance  of  iron  sud- 
denly fractured  ;  the  two,  combined  in  the  same  specimen, 
are  shown  in  iron  bolts  partly  converted  into  steel. 

22.  The  little  additional  time  required  in  testing  those 
specimens  whose  rate  of  elongation  was  noted  had  no  inju- 
rious effect  in  lessening  the  amount  of  breaking  strain,  as 
imagined  by  some. 

28.  The  rate  of  elongation  varies  not  only  extremely  in  dif- 
ferent qualities,  but  also  to  a  considerable  extent  in  speci- 
mens of  the  same  brand. 

24.  The  specimens  were  generally  found  to  stretch  equally 
throughout  their  length  until  close  upon  rupture,  when  they 
more  or  less  suddenly  drew  out,  usually  at  one  part  only, 
sometimes  at  two,  and,  in  a  few  exceptional  cases,  at  three  dif- 
ferent places. 

25.  The  ratio  of  ultimate  elongation  may  be  greater  in 
short  than  in  long  bars  in  some  descriptions  of  iron,  whilst 
in  others  the  ratio  is  not  affected  by  difference  in  the 
length. 

26.  The  lateral  dimensions  of  specimens  forms  an  impor- 
tant element  in  comparing  either  the  rate  of,  or  the  ultimate 
elongations — a  circumstance  which  has  been  hitherto  over- 
looked. 

27.  Iron  bolts,  case-hardened,  bore  a  less  breaking  strain 


STEENGTH  OF  WROUGHT  IRON. 


151 


than  when  wholly  iron,  owing  to  the  superior  tenacity 
of  the  small  proportion  of  steel  being  more  than  counter- 
balanced by  the  greater  ductility  of  the  remaining  portion  of 
iron. 

28.  Iron  highly  heated  and  snddenly  cooled  in  water  is 
hardened,  and  the  breaking  strain,  when  gradually  applied, 
increased,  but  at  the  same  time  it  is  rendered  more  liable  to 
snap. 

29.  Iron,  like  steel,  is  softened,  and  the  breaking  strain  re- 
duced by  being  heated  and  allowed  to  cool  slowly. 

30.  Iron,  subjected  to  the  cold-rolling  process,  has  its 
breaking  strain  greatly  increased  by  being  made  extremely 
hard,  and  not  by  being  "  consolidated,"  as  previously  sup- 
posed. 

31.  Specimens  cut  out  of  crank-shaft  are  improved  by 
additional  hammering. 

32.  The  galvanizing  or  tinning  of  iron  plates  produces  no 
sensible  effects  on  plates  of  the  thickness  experimented  on. 
The  results,  however,  may  be  different  should  the  plates  be 
extremely  thin. 

33.  The  breaking  strain  is  materially  affected  by  the  shape 
of  the  specimen.  Thus  the  amount  borne  was  much  less  when 
the  diameter  was  uniform  for  some  inches  of  the  length  than 
when  confined  to  a  small  portion — a  peculiarity  previously 
miascertained  and  not  even  suspected. 

34.  It  is  necessary  to  know  correctly  the  exact  conditions 
under  which  any  tests  are  made,  before  we  can  equitably 
compare  results  obtained  from  different  quartei-s. 

35.  The  startling  discrepancy  between  experiments  made 
at  the  Koyal  Arsenal,  and  by  the  writer,  is  due  to  the  differ- 
ence in  the  shape  of  the  resjDCctive  specimens,  and  not  to  the 
difference  in  the  two  testing  machines. 

36.  In  screwed  bolts  the  breaking  strain  is  found  to  be 
greater  when  old  dies  are  used  in  their  formation  than  when 
the  dies  are  new,  owing  to  the  iron  becoming  harder  by  the 
greater  pressure  I'equired  in  forming  the  screw  thread  when 
the  dies  are  old  and  blunt,  than  when  new  and  sharp. 

37.  The  strength  of  screw-bolts  is  found  to  be  in  propor- 
tion to  their  relative  areas,  there  being  only  a  slight  difference 
in  favor  of  the  smaller  compared  with  the  larger  sizes,  instead 
of  the  very  material  difference  previously  imagined. 

38.  Screwed  bolts  are  not  necessarily  injured  although 
strained  nearly  to  their  breaking-point. 

39.  A  great  variation  exists  in  the  strength  of  iron  bars 
which  have  been  cut  and  welded ;  wdiilst  some  bear  almost  as 


152 


CIVIL  ENGINEERING. 


much  as  the  uncut  bar,  the  strength  of  others  is  reduced  fully 
a  third. 

40.  Iron  is  injured  by  being  brought  to  a  white  or  welding 
heat  if  not  at  the  same  time  hammered  or  rolled. 

41.  The  breaking  strain  is  considerably  less  when  the  strain 
is  applied  suddenly  instead  of  gradually,  though  some  have 
imagined  that  the  reverse  is  the  case. 

44.  The  contraction  of  area  is  also  less  when  the  strain  is 
suddenly  applied. 

43.  The  breaking  strain  is  reduced  when  the  iron  is  frozen; 
with  the  strain  gradually  applied,  the  difference  between  a 
frozen  and  unfrozen  bolt  is  lessened,  as  the  iron  is  warmed  by 
the  drawing  out  of  the  specimen. 

44.  The  amount  of  heat  developed  is  considerable  wlien  the 
specimen  is  suddenly  stretched,  as  shown  in  the  formation  of 
va])or  from  the  melting  of  the  layer  of  ice  on  one  of  the  spe- 
cimens, and  also  by  the  surface  of  others  assuming  tints  of 
various  shades  of  blue  and  orange,  not  only  in  steel,  but  also, 
although  in  a  less  marked  degree,  in  iron. 

45.  The  specific  gravity  is  found  generally  to  indicate 
pretty  correctly  the  quality  of  specimens. 

46.  The  density  of  iron  is  deor eased  by  the  process  of  wire- 
drawing, and  by  the  similar  process  of  cold-rolling,  instead  of 
increased^  as  previously  imagined. 

47.  The  density  in  some  descriptions  of  iron  is  also  de- 
creased by  additional  hot-rolling  in  the  ordinary  way ;  in  others 
the  density  is  very  slightly  increased. 

48.  Tlie  density  of  iron  is  decreased  by  being  drawn  out 
under  a  tensile  strain,  instead  of  increased,  as  believed  by 
some. 

The  breaking  strain  per  square-inch  of  Avrought  iron  is 
generally  stated  to  be  about  twenty-five  tons  for  bars,  and 
twenty  tons  for  plates.  This  corresponds  very  nearly  with 
the  results  of  the  writer's  experiments,  of  which  the  follow- 
ing table  presents  a  condensed  smnmary : — 

Highest,  lbs. 

188.  Bars,  rolled  08,848 

72.  Ang"le-iron,  etc  68,715 

1 67 .  Plates,  leng-thwaj'-s  62, 544 

160.  Plates,  crossways  60,756 

Although  the  hreaJcing  strain  is  generally  assumed  to  be 
about  twenty-five  tons  for  bars,  and  twenty  tons  for  plates, 
very  great  difference  of  opinion  exists  as  to  the  amount  of 
working  strain,  or  the  load  which  can  with  safety  be  applied 


Lowest,  lbs.  Mean.  lbs.  Tons, 

44,584  57,555  =25f 

37.909  54.729  =24^ 

37,474  50,787  )  _oi8 

32,450  40,171  )  — 


STRENGTH  OF  WROUGHT  IRON. 


153 


in  actual  practice.  The  latter  is  variously  stated  at  from  a 
third  to  a  tenth.  It  will  be  observed  that  whilst  much  dis- 
cussion has  arisen  as  to  the  amount  of  working  strain,  or  the 
ratio  the  load  should  bear  to  that  of  the  breaking  strain,  the 
important  circumstance  of  the  quality  of  the  iron,  as  in- 
fluencing the  working  strain,  has  been  overlooked.  The  Board 
of  Trade  limits  the  strain  to  5  tons,  or  11,200  lbs.  per  square 
inch. 

It  must  be  abundantly  evident,  from  the  facts  wliich  have 
been  produced,  that  the  breaking  strain,  when  taken  alone, 
gives  a  false  impression  of,  instead  of  indicating,  the  real 
quality  of  the  iron,  as  the  experiments  which  have  been  in- 
stituted reveal  the  somewhat  startling  fact,  that  frequently 
the  inferior  kinds  of  iron  actually  yield  a  higher  result  than 
the  superior.  The  reason  of  this  dilference  was  shown  to  be 
due  to  the  fact  that,  whilst  the  one  quality  retained  its  ori- 
ginal area,  only  very  slightly  decreased  by  the  strain,  the 
other  was  reduced  to  less  than  one-half.  N(jw,  surel}^  this 
variation,  hitherto  iinaccountably  completely  overlooked,  is  of 
importance  as  indicating  the  relative  hardness  or  softness  of 
the  material,  and  thus,  it  is  submitted,  forms  an  essential  ele- 
ment in  considering  the  safe  load  that  can  be  practically 
applied  in  various  structures.  It  must  be  borne  in  mind  that 
although  the  softness  of  the  material  has  the  effect  of  lessen- 
ing the  amount  of  the  hreaking  strain,  it  has  the  very  opposite 
effect  as  regards  the  workmg  strain.  This  holds  good  for 
two  reasons :  first,  the  softer  the  iron  the  less  liable  it  is  to 
snap  ;  and  second,  fine  or  soft  iron,  being  more  uniform  in 
quality,  can  be  more  depended  upon  in  practice.  Hence  the 
load  which  this  description  of  iron  can  suspend  with  safety 
may  approach  much  more  nearly  the  limit  of  its  breaking 
strain  than  can  be  attemj)ted  with  the  harder  or  coarser  sorts, 
where  a  greater  margin  must  necessarily  be  left. 

Special  attention  is  now  solicited  to  the  practical  use  that 
may  be  made  of  the  new  mode  of  comparison  introduced  by 
the  writer,  viz.,  the  hreaking  strain  jper  square  inch  of  the 
fractured  area  of  the  specimen^  instead  of  the  hreaking  strain 
2?er  square  inch  of  the  original  area. 

As  a  necessary  corollary  to  what  he  has  just  endeavored  to 
establish,  the  writer  now  submits,  in  addition,  that  the  work- 
ing sti-ain  should  be  in  proportion  to  the  breaking  strain  per 
square  inch  of  fractured  area,  and  not  to  the  breaking  strain 
per  square  inch  of  original  area,  as  heretofore.  He  does  not 
presume  to  say  what  that  ratio  should  be,  but  he  fully  main- 
tains that  some  kinds  of  iron  experimented  on  by  him  will 


154 


CIVIL  ENGINEERING. 


sustain  with  safety  more  than  double  the  load  that  others  can 
suspend,  especially  in  circumstances  Avhere  the  load  is  un- 
steady, and  the  structure  exposed  to  concussions,  as  in  a  ship, 
or  to  Anbrator}^  action,  as  in  a  raihvay  bridge. 

£69.  Resistance  to  Compressive  Strain.  But  few  ex- 
periments have  been  publislied  on  the  resistance  of  this 
material  to  compression.  Rondelet  states  that  it  commences 
to  yield  under  a  pressure  of  about  70,800  lbs.  per  square  inch, 
and  that  when  the  altitude  of  the  specimen  tried  is  greater 
than  three  times  the  diameter  of  the  base  it  yields  by  bending. 
Mr.  Ilodgkinson  states  that  the  circumstances  of  its  rupture 
from  crushing  indicate  a  law  similar  to  what  obtains  in  cast 
iron. 

The  same  rule  for  proportioniug  the  working  strain- to  the 
crushing  strain  is  followed  in  wrought  iron  subjected  to  com- 
pression as  in  cast  iron. 

Resistance  to  a  Transverse  Strain.  The  following 
tables  exhibit  the  circumstances  of  deflection  from  a  transverse 
strain  on  bars  laid  on  horizontal  supports  ;•  the  weight  being 
applied  at  the  middle  of  the  bar. 

The  table  I.  gives  the  results  on  bars  2  inches  square,  laid 
on  supports  33  inches  asunder ;  table  II.  the  results  on  bars 
2  inches  deep,  1.9  in.  broad,  bearing  as  in  table  I. 

Table  L  Table  II. 


Weight  in  tons. 

Deflections  in  | 
inches  for  each 
half  ton. 

Weight-in  tons. 

Deflections  in 
inches  for  each 
half  ton. 

.75 

.020 

.250 

1.00 

.020 

.50 

.016 

1.50 

.020 

1.00 

.022 

2.00 

.030 

1.50 

.020 

2.50 

.020 

2.00 

.026 

3.00 

Set 

2.25 

.018 

2.50 

.026 

2.75 

.038 

3.00 

.092 

The  above  experiments  were  made  by  Professor  Barlow, 
and  publislied  in  his  re])ort  already  cited.  lie  remarks  on 
the  results  in  Table  II.,  that  the  elasticity  was  injured  by  2.50 
tons  and  destroyed  by  3.00  tons. 

370.  Trials  were  made  to  ascertain  mechanicallv  the  posi- 
tion of  the  neutral  axis  on  the  cross  section.  Professor  Bar- 
low remarks  on  these  trials,  that  "  the  measurements  obtained 
in  these  experiments  being  tension  1.6,  compression  0.4,  giv- 


9 


STRENGTH  OF  WKOUGIIT  IRON. 


155 


ing  exactly  the  ratio  of  1  to  4  in  rectangular  bars.  These  re- 
sults seem  the  most  positive  of  any  hitherto  obtained ;  still 
there  can  be  little  doubt  this  ratio  varies  in  iron  of  different 
qualities;  but  looking  to  the  preceding  experiments,  it  is 
probably  always  from  1  to  3,  to  1  to  5." 

371.  Effects  of  Time  on  the  Elongation  of  Wrought  Iron 
from  a  Constant  Strain  of  Extension.  M.  Yicat  has  given, 
in  the  Annales  de  Chimie  et  de  Physique,  vol.  54,  some  ex- 
periments on  this  point,  made  on  iron  wires  which  had  not 
been  annealed,  by  subjecting  four  wires,  respectively,  to 
strains  amounting  to  the  J,  the  |-,  the  -I,  and  f  of  their  tensile 
strength,  during  a  period  of  33  months. 

From  the  results  of  these  experiments  it  appears,  that  each 
wire,  immediately  upon  the  application  of  the  strain  to  which 
it  was  sul^jected,  received  a  certain  amount  of  extension. 

The  first  wire,  which  was  subjected  to  a  strain  of  ^\h.  its 
tensile  strength,  was  found  at  the  end  of  the  time  in  question 
not  to  have  acquired  any  increase  of  extension. 

The  second,  submitted  to  -g-d  its  tensile  strength,  was  elon- 
gated 0.027  in.  per  foot,  independently  of  the  elongation  it  at 
first  received. 

The  third,  subjected  under  like  circumstances  to  a  strain  of 
Jth  its  tensile  strength,  was  elongated  0.40  in.  per  foot,  be- 
sides its  first  elongation. 

The  fourth,  similarly  sul^jected  to  |ths  the  tensile  strength, 
was  elongated  0.061,  besides  its  first  elongation. 

From  observations  made  during  the  experiments,  it  was 
found  that,  reckoning  from  the  time  wdien  the  first  elongations 
took  23lace,  the  rapidity  of  the  subsequent  elongations  was 
nearly  proportional  to  the  times ;  and  that  the  elongations 
from  strains  greater  than  ^th  the  tensile  strength  are,  after 
equal  times,  nearly  proportional  to  the  strains. 

M.  Yicat  remarks  in  substance,  upon  the  results  of  these  ex- 
periments, that  iron  wire,  when  not  aimealed,  commences  to 
exhibit  a  permanent  set  when  subjected  to  a  strain  between  the 
\  and  i  of  its  tensile  strength,  and  that  therefore  it  is  rendered 
probable  that  the  wire  ropes  of  a  suspension  bridge,  whi(,'h 
should  be  subjected  to  a  like  strain,  would,  when  the  vibratory 
motion  to  which  such  structures  are  liable  is  considered,  yield 
constantly  from  year  to  year,  until  they  entirely  gave  way. 

M.  Yicat  further  remarks,  in  substance,  that  the  measure  of 
the  resistance  offered  by  materials  to  strains  exerted  only  some 
minutes,  or  hours,  is  entirely  relative  to  the  duration  of  the 
experiments.  To  ascertain  the  absolute  measure  of  this  re- 
sistance, which  should  serve  as  a  guide  to  the  engineer,  the 


150 


CIVIL  ENGINEERING. 


materials  ought  to  be  subjected  for  some  months  to  strains  ; 
while  observations  should  be  made  during  this  period,  with 
accurate  instruments,  upon  the  manner  in  which  they  yield 
under  these  strains. 

The  following  tahles,  on  the  comparative  strength  of  iron, 
steel  and  hemp  rope  are  taJcen  from  Btoneifs  work  on  the 
Theory  of  Strains,  Vol.  II.  The  weights  are  given  in 
English  units. 


HEMP. 

IKON. 

STEEL. 

EQUIVALENT  STRENGTH. 

cT 
o 

u 

a 

0 

c 

s  . 

a 

6 

2 

^  . 

•s  § 

tc 

\l 

ei 

?  -d 

^  0 

.s  «• 

^  -e 

11 

«2 

0  y 

11 

b 







5 

H 

2J 

2 

1 

1 

0 

2 

n 

H 

i 

i* ' 

9 

3 

3| 

4 

If 

2 

12 

4 

If 

2i 

'ii* 

'ii' 

15 

5 

A  1 

0 

3 

18 

0 

2 

3i 

"if' 

'2' 

21 

7 

'5i' 

"7* 

2^ 

4 

If 

24 

8 

2i 

4i 

27 

9 

'e" ' 

"9' 

2f 
2i 

5 

5i 

'ii' 

'd" 

0  CO 
CO  CO 

10 
11 

'ei' 

*i6" 

2f 
2f 

6 

2' ' 

'si' 

36 

13 

Oi 

2i 

4 

39 

13 

'7** 

'12" 

2-5 
3 

7 

4i 

42 
45 

14 
15 

'h\ 

'ii' 

3i 
3i 

8 

Si 

5" ' 

48 
51 

16 
17 

's" 

'iG 

31 

9 

'2i' 

'si' 

54 

18 

3^ 
3f 

10 

2f 

6 

60 

20 

'si' 

'is 

11 

2f 

66 

22- 

3f 

12 

72 

24 

'oi' 

'22 

3^ 

13 

'3i* 

's" 

78 

26 

10 

26 

4 

14 

84 

28 

4i 

15 

'si' 

'9" 

90 

30 

ii' " 

"36* 

41 

16 

96 

33 

4i 

18 

'si' 

io" 

108 

36 

12" 

'34 

4f 

20 

3f 

12 

120 

40 

STRENGTH  OF  WEOUGHT  lEON.  157 


BESSEMER  STEEL,  MADE  FROM  RAIL-ENDS  BX  MARSH  &  CO.     NOT  TEMPERED. 


No. 

Breaking 
strain. 

strength 
per  square 
inch. 

Feet  in  the 
lb. 

stretch. 

Per  cent,  of 
length. 

Length. 

Drawn  from. 

a 
D 

118  471 

10.15 

li 

2.4 

o.\j  t  oo 

4-6. 

6 

110503 

10  28 

4 

8 

1  3 

4.975 

4-6! 

t 

3038 

114^549 

11.*  21 

1 

1.'05 

4-  Qon^ 

t.  i/uuo 

A   7  •  Inro'P  7 

t: —  1   ^  IdlgC  i 

< 

3136 

122,880 

11.6 

H 

1.8 

5.1 

4-7. 

Q 
O 

2135 

109,034 

15.2 

1.04 

4.9844 

4-8. 

Q 

«7 

2184 

127,000 

17.3 

0.35 

4. 8646 

4-6  and  6-9 

Q 

1904 

109,770 

17.14 

f 

0.65 

4.823 

4-6  and  6-9. 

10 

1694 

117,567 

20.6 

f 

1.2 

4^8375 

4-10  j  no  annealing" 

10 

1610 

111,718 

20.6 

f 

0.6 

4.8375 

4-10  \  between  hard  drawn. 

10 

1834 

130,493 

21.16 

1 

1 

4.96 

4-7  and  7-10  not  drawn 
hard. 

n 

1407 

121,900 

25.7 

0.8 

4.833 

4-8  and  8-11   not  drawn 
hard. 

12 

1015 

121,679 

32.6 

i 

1.5 

4.6094 

4-7  ;  7-10  and  10-12. 

13 

952 

131,055 

40.9 

0.8 

5.1106 

4-7  ;  7-10  and  10-13. 

14 

630 

114,508 

54 

f 

1.2 

5.073 

4-7 ;  7-10  ;  10-12  and  12-14. 

15 

560 

170,740 

62.5 

i 

0.85 

4  8854 

4-7  ;  7-10 ;  10-12  and  13-15. 

18 

466 

130,286 

83.17 

1 

0.6 

4.947 

GERMAN  PUDDLED  STEEL.     FALKENWORTH  ROCHER  &  CO.     NOT  TEMPERED. 


No. 

Breaking 
strain. 

strength 
per  square 
inch. 

Feet  in  the 
lb. 

stretch. 

Per  cent,  of 
length. 

Length. 

Drawn  from. 

8 

2226 

110,200 

14.75 

1 

0.6 

5.0625 

4-8. 

Drawn  in  Germany. 

9 

1778 

106,900 

17.87 

0.82 

5  026 

4-9 

9 

1820 

108,700 

17.75 

f 

1.2 

4.9948 

4-9 

CAST-STEEL,  PIANO  WIRE.     (M.  POHLMANN,  NUREMBERG.) 


No. 

Breaking 
Strain. 

Strength  per 
sq.  inch. 

Feet  v.\  the 
lb. 

stretch 

Per 
cent,  of 
length. 

Length. 

Drawn  from. 

14 

1624 

302,500 

55.4 

H 

1.8 

5.1944 

drawn  wet,  no  an- 

14^ 

1400 

299,225 

63.5 

2.6 

4.96 

nealing  below 

15 

1008 

263,117 

70.8 

1^6 

1.8 

4.69 

10. 

15 

1078 

270,000 

74.5 

1.6 

4.656 

16 

774 

249,7('0 

96.0 

8 

1.6 

4.5 

16i 

812 

283,r>20 

103.8 

1^6 

2.0 

4.865 

m 

784 

275,52.-) 

104.8 

f 

1.2 

4.9 

m 

763 

261,576 

102.0 

1.4 

4.78 

158  CIVIL  ENGINEERING. 


CAST-STEEL.     (JOIINSON,  NFrHEW.) 


No. 

Breaking 
Strain. 

strength  per 
sq.  inch. 

Feet  in  the 
lb. 

stretch. 

Per 
cent,  of 
length. 

Length, 

DrawTi  from. 

GO  00  00 

3220 
3202 
3100 

158,823 
100,000 
155,400 

14.67 

14.5 

14.6 

1| 
If 

2.2 

3 
2 

5.043 

5.0143 

5.026 

4-8  ) 

4-8  >•  Tempered. 
4-8) 

CAST- STEEL.     (WEBSTER,  HORSFALL.) 

No. 

Breaking 
Strain. 

strength  per 
sq.  inch. 

Feet  in  the 
lb. 

stretch. 

Per 
cent,  of 
length. 

Length. 

Drawn  from. 

9 
9 
9 

10 

285G 
2812 
2842 
1988 

167,601 
166,122 
168,506 
150,560 

17.6 
17.6 
17.6 
22.6 

H 

n 
n 

i 

2 

1.8 
1.8 
1.4 

4.96 
4.96 
4.96 
4.927 

1  4-8,  then  tem- 
j- pared  and  fin- 
j  ished  in  1  hole. 

The  following  results  were  computed  from  experiments  by 
the  late  J.  A.  Roebling,  the  eminent  engineer  of  the  Niagara, 
Cincinnati  and  other  suspension  bridges,  on  the  comparative 
strength  of  iron-wire  rope  and  of  hemp  rope.  The  breaking 
weight  being  in  tons  of  2,000  lbs. 


No. 

Circumference 
of  wire  rope  in 
inches. 

Area  of  section 
in  sq,  inches. 

Trade  number. 

CircTimfcrence 
of  hemp  rope 
iu  inches. 

Area  of  section 
in  sq.  inches. 

Tearing  f 
square  inc 

"Wire 
roi)e. 

•train  per 
h  in  tons. 

Hemp 
rope. 

1 

4.9 

1.9 

4 

12 

11.45 

22.8 

3.8 

2 

3.91 

1.22 

6 

9.5 

7.18 

22.3 

3.78 

3 

2.98 

0.7 

8 

7 

3.9 

22.8 

4.1 

4 

4.00 

1.27 

12 

10 

7.95 

23.0 

3.77 

5 

2.98 

0.7 

15 

7.25 

4.18 

22.8 

3.82 

JVofe.  ]N"os.  1,  2,  3,  were  made  of  what  is  known  as  fine 
w^ire ;  'Nos.  4,  5,  of  coarse  wire. 


372.  Effects  of  Temperature  on  the  Tensile  Strength 
of  Wrought  Iron.  The  experiments  made  under  the  direc- 
tion of  the  Franklin  Institute,  already  noticed,  have  developed 
some  very  curious  facts  of  an  anomalous  character,  with  re- 
spect to  the  effect  of  au  increase  of  temperature  upon  the 


STRENGTH  OF  WEOUGIIT  IKON. 


159 


strength  of  wrought  iron.  It  was  found  that  at  high  degrees 
of  heat  the  tensile  strength  was  grea,ter  up  to  a  certain  point 
than  was  exhibited  by  the  same  iron  at  ordinary  temperatures. 
The  Sub-committee  in  their  Report  remark  :  "  This  circum- 
stance was  noted  at  .212°,  392°,  and  572°,  rising  by  steps  of 
180°  each  from  32°,  at  which  last  point  some  trials  have  been 
made  in  melting  ice.  At  the  highest  of  these  points,  however, 
it  was  perceived  that  some  specimens  of  the  metal  exhibited 
but  little,  if  any,  superiority  of  strength  over  that  which  they 
had  possessed  when  cold,  while  others  allowed  of  being  heated 
nearly  to  the  boiling-point  of  mercury,  before  they  manifested 
any  decided  indications  of  a  weakening  effect  from  increase 
of  temperature." 

"  It  hence  became  a]3parent  that  any  law,  taking  for  a 
basis  the  strength  of  iron  in  its  ordinary  condition,  and  at 
common  temperatures,  must  be  liable  to  great  uncertainty,  in 
regard  to  its  application  to  different  specimens  of  the  metal. 
It  was  evident  that  the  anomaly  above  referred  to  must  be 
only  apparent,  and  that  the  tenacity  actually  exhibited  at  572°, 
as  well  as  that  which  prevails  while  the  iron  is  in  the  state  in 
which  it  was  left  by  foi'ging  or  rolling,  must  be  below  its 
maximum  tenacity." 

From  the  experiments  made  upon  several  bars  of  the  same 
iron,  it  appeared  that  their  "  maximum  tenacity  was  15.17  per 
cent,  greater  than  their  mean  strength  when  tried  cold." 

Calculating  the  maximum  tenacit}^  in  other  experiments 
from  this  standard,  the  Sub-committee  have  drawn  up  the 
following  table  exhibiting  the  relations  between  diminutions 
from  the  maximum  tenacity  and  the  degrees  of  temperature 
by  which  they  are  caused,  from  which  the  curve  representing 
the  law  of  these  relations  can  be  constructed. 

The  Sub-committee  remark  on  the  construction  of  the  above 
table :  "  As  some  of  the  experiments  which  furnished  the 
standards  of  comparison  for  strength  at  ordinary  temperatures 
were  made  at  80°,  and  as  at  this  point  small  variations  with  re- 
spect to  heat  appear  to  affect  but  very  slightly  the  tenacity  of 
iron,  it  was  conceived  that  for  practical  purposes,  at  least,  the 
calculations  might  be  commenced  from  that  point." 

It  will  be  found  that  with  the  exception  of  a  slight  anoma- 
ly between  520°  and  570°,  amounting  to  — .08,  the  numbers 
expressing  the  ratios  between  the  elevations  of  temperature, 
and  the  diminutions  of  tenacity,  constantly  increase  until  we 
reach  932°,  at  which  it  is  2.97,  and  that  from  this  point  the 
ratio  of  diminution  decreases  to  the  limits  of  our  range  of 
trials,  1317°,  where  it  is  2.14.    It  will  also  be  observed,  that 


160 


CIYIL  ENGINEERING. 


the  diminution  of  tenacity  at  932°,  where  the  law  changes 
from  an  increasing  to  a  decreasing  rate  of  diminution,  is 
almost  precisely  one-third  of  the  total,  or  maximum  strength 
of  the  iron  at  ordinary  temperatures." 


TABLE. 


No.  of  the  com- 
parison. 

ObserTed  tem- 
peratures. 

Observed  tem- 
peratures— 80°. 

Observed  dimi- 
nution of  te- 
nacity. 

Power  of  the  temperature 
which  represents  the  di- 
minution of  tenacity  at 
each  point. 

1 

520° 

440° 

.0738 

2.25 

3 

570 

490 

.0869 

2.17  ' 

3 

596 

516 

.0899 

2.38 

4 

662 

582 

.1155 

2.67 

5 

770 

690 

.1627 

2.85 

6 

824 

744 

.2010 

2.94 

7 

932 

852 

.3324 

2.97 

8 

1030 

950 

.4478 

2.92 

9 

1111 

1031 

.5514 

2.63 

10 

1155 

1075 

.6000 

2.60 

11 

1237 

1157 

.6622 

2.41 

12 

1317 

1237 

.7001 

2.14 

Mean  2.58 

From  the  mean  of  all  the  rates  in  the  above  table  the  fol- 
lowing rule  is  deduced  :  "  the  thirteeiith  joower  of  the  temper- 
ature above  80°  is  pTOj)ortionate  to  the  fifth  power  of  the 
diminution  from'  the  maximum  tenacity.''^ 

Professor  W.  K.  Johnson,  a  member  of  the  Sub-committee, 
has  since  applied  the  results  developed  in  the  preceding  ex- 
pei'iments  to  practical  purposes,  in  increasing  the  tenacity  of 
wrought  iron  by  subjecting  it  to  tension  under  a  high  degree 
of  temperature,  before  using  it  for  purposes  in  which  it  will 
have  to  undergo  considerable  strains,  as,  for  example,  in  chain 
cables,  etc. 

This  subject  was  brought  by  Prof.  Johnson  before  the 
Board  of  Navy  Commissioners  in  1841 ;  subsequently,  experi- 
ments were  made  by  him  under  direction  of  the  Navy  De})art- 
ments  the  results  of  which,  as  exhibited  in  the  following 
table,  were  published  in  the  Senate  Fuhlic  Documents  (1), 
28i5A  Congress,  M  Session,  p.  641.    Dec.  3,  1844.  , 

Prof.  Johnson  in  his  letter  remarks  :  "It  will  be  observed 
that  in  these  experiments  the  temperature  has,  with  a  view  to 
economy  of  time,  been  limited  to  400°,  whereas  the  best 


STRENGTH  OF  WEOTJGHT  lEON.  161 

effects  of  the  process  have  generally  been  obtained  heretofore 
when  the  heat  has  been  as  high  as  575°." 


Table  of  the  Effects  of  Thermo-tension  on  the  Tenacity 
and  Elongation  of  Wrought  Iron. 


KIND  OF  IKON. 

Strength 
of  cold. 

strength  af- 
ter treating 
with  Ther- 
mo-tension. 

Gain  of 
length. 

Gain  of 
strength  by 
the  treat- 
ment. 

Total  gain 
of  value. 

Tredegar,  No.  1,  round  iron 

Do.  do. 
Tredegar,  square  bar  iron 
Tredegar,  No.  3,  round  iron 
Salisbury,  round  (Ames') 

60 
60 
60 
58 

105.87 

71.4 
72.0 
67.2 
68.4 
121.0 

6.51 

6.51 

6.77 

5.263 

3.73 

19.00 
20.00 
12.00 
17.93 
14.29 

25.51 
26.51 
'  18.77 
23.19 
18.02 

Mean, 

5.75 

16.64 

22.40 

From  the  experiments  of  Mr.  Kirkaldy  it  appears  that 
"wrought  iron  is  injured  by  being  brought  to  a  white  heat  if 
not  at  the  same  time  hammered  or  rolled." 

Resistance  of  Wrought  Iron  and  Steel  to  a  Shearing  Strain, 
From  the  experiments  of  Mn  Clark  on  plates  joined  by  a 
single  wroiight-iron  rivet,  and  those  of  Mr.  Kirkaldy  on  steel 
rivets,  it  appears  that  the  resistance  to  a  shearing  strain  of  the 
former  was  very  nearly  equal  to  its  tensile  strength  ;  and  for 
the  latter  that  it  was  about  three-fourths  of  its  tensile 
strength. 

373.  Resistance  of  Iron  Wire  to  Impact.  The  follow- 
ing table  of  experiments  gives  the  results  obtained  by  Mr. 
Hodgkinson,  by  suspending  an  iron  ball  at  the  end  of  a  wire 
(diameter  ISTo.  17),  and  letting  another  iron  ball  impinge 
upon  it  from  different  altitudes.  The  suspended  and  imping- 
ing balls  had  holes  drilled  through  them,  through  which  the 
wire  passed.  A  disk  of  lead  was  placed  on  the  suspended 
ball  to  receive  the  blow,  and  lessen  the  recoil  from  elasticity. 

The  following  observations  are  made  by  Mr.  Hodgkinson : 
"  To  ascertain,  the  strength  and  extensibility  of  this  wire,  it 
was  broken  in  a  very  careful  experiment  with  252|-  lbs.,  sus- 
pended at  its  lower  end,  and  laid  gradually  on.  And  to  ob- 
tain the  increment  of  a  portion  of  the  wire  (length  24  ft.  8  in.) 
when  loaded  by  a  certain  weight,  it  had  139  lbs.  hung  at  the 
bottom,  and  when  89  lbs.  were  taken  off  the  load,  the  wire 
decreased  in  length  .39  inch. 
11 


162 


CIVIL  ENGINEKRmG. 


TABLE. 


'L 

•ft  "3 

<U  p.  C3 

Height  fallen  through  by 

■a 

Weight  < 
king  b 

striking  ball. 

Wire  br( 
ball  fa 
throng 

Bemarka. 

s 
1^ 

ft.  in. 

lbs.  oz. 

lbs.  oz. 

25  0 

5  14 

0  9 

2,  2X,  3,  3>f,  4, 

No  lead. 

(repeated)  2^,  3,  3X,  4,  4^, 

24  0 

e"  0 

10  1 

7, 

The  wire  usu- 

(repeated with  fresh  wire,)  6, 

ally  broke  near 

44  0 

1,  2,  3,  4,  5,  6,  ^M,  7, 
6,  6j^,  7,  7^,  8,  8X,  9, 
8,  8X,  9,  9>,  10,  lOX, 

the  point  of  im- 

80 8 

9X 

pact,  and  it  was 
adjusted  to  its 

89  0 

11 

125  0 

8,  8M,  9,  9X,  10, 

10)^ 

length,  if  fresh 

40  0 

10  1 

3,  4  inches, 

5  inches 

wire    were  not 

80  8 

2,  3,  4,  5,  6  inches, 

7  do. 

used  by  a  reserve 
at  the  top. 

89  0 

4,  5  inches, 

6  do. 

Broke  one  inch 

24  8 

85  0 

44  0 

2  inches. 

3  do. 

from  top. 

"  Should  it  be  suggested  that  the  wire  by  being  frequently 
impinged  upon  would  perhaps  be  much  weakened,  the  author 
would  beg  to  refer  to  a  paper  of  his  on  Chain  Bridges,  3fan- 
chester  Memoirs^  2d  series,  vol.  5,  where  it  is  shown  that  an 
iron  wire  broken  by  pressure  several  times  in  succession  is 
very  little  weakened,  and  will  nearly  bear  the  same  weight  as 
at  first." 

"  The  fii*st  of  the  preceding  experiments  on  wires  are  the 
only  ones  from  which  the  maximum  can,  with  any  approach 
to  certainty,  be  inferred  ;  and  we  see  from  them  that  the  wire 
resisted  the  impulsion  with  the  greatest  effect  when  it  was 
loaded  at  bottom  with  a  weight,  wdiich,  added  to  that  of  the 
striking  body,  was  a  little  more  than  one-third  of  the  weight 
that  would  break  the  wire  by  pressure." 

"  From  these  experiments  generally,  it  appears  that  the  wire 
was  weak  to  bear  a  blow  when  lightly  loaded." 

"  These  last  experiments  and  remarks,  and  some  of  the  pre- 
ceding ones  "  (on  horizontal  impact),  "  show  clearly  the  benefit 
of  giving  considerable  weight  to  elastic  structures  subject  to 
impact  and  vibration." 

374.  Resistance  to  Torsion  of  Wrought  and  Cast  Iron. 
— The  following  table  exhibits  the  results  of  experiments 
made  by  Mr.  Dunlop,  at  Glasgow,  on  round  bars  of  wrought 
iron.  The  twisting  weights  were  applied  with  an  arm  of  lever 
14  feet  2  inches. 


STRENGTH  OF  STEEL. 


163 


liCn^li  of  bars 
in  inches. 

Diameter  of  bars 
in  inches. 

Weight  in  lbs.  pro* 
ducing  rupture. 

9 

<cOU 

2i 

384 

3 

2i 

408 

3 

2f 

700 

4 

3i 
3i 

1170 

5 

1240 

5 

3f 

1663. 

5 

4 

1938 

6 

4i 

2158 

Table  of  Experiments  made  by  Mr.  G.  Rennie  ujpon  Cast 
and  Wrought  Iron,  Weight  ajpjplied  at  an  arm  of  lever  of 
2  feet. 


Length  of 

Size  of 

Mean 

break- 

MATERIAL. 

blocks  in 

sectional 

ing 

weight 

inches. 

area. 

in  lbs. 

lbs. 

oz. 

0 

i 

9 

15 

0 

i 

10 

10 

i 

7 

3 

f 

I 

8 

1 

(t  u 

1 

8 

8 

((  n 

i 

10 

1 

f 

i 

8 

9 

li  u 

1 

i 

8 

5 

((  (( 

6 

i 

9 

12 

0 

93 

13 

4( 

0 

i 

74 

((  u 

10 

i 

52 

0 

i 

10 

2 

"  {Swedi&k)  

0 

9 

8 

YII. 

STRENGTH  OF  STEEL. 

375.  From  experiments  made  in  Sweden  by  a  government 
commission  it  appears  that  both  the  ductility  and  the  strength 
of  steel  and  iron  are  influenced  by  the  amount  of  carbon  they 
contain. 


164 


CIVIL  ENGINEEEINO. 


The  experiments  show  that  the  hardest  material  has  the 
greatest  strength  both  before  and  after  a  permanent  set  has 
taken  place  from  the  force  employed  ;  but  its  ductility  is  also 
the  least.  The  Bessemer  steel  in  these  experiments  gave  the 
same  results  as  the  other  processes  for  obtaining  steel,  the 
same  pig  iron  being  used  in  each  case. 

The  limit  for  the  amount  of  carbon  for  the  Bessemer  steel 
is  from  1.2  to  1.5  per  cent.  AVith  a  larger  amount  both  the 
strength  and  ductility  was  found  to  decrease.  When  the 
amount  of  carbon  does  not  exceed  0.4  per  cent,  the  ductility 
of  Bessemer  steel  is  about  the  same  as  puddled  iron  from  the 
same  pig  iron,  and  as  it  is  not  only  much  stronger  but  more 
dense  and  homogeneous  than  the  puddled  material,  it  is  de- 
cidedly superior  for  railway  purposes. 

From  the  experiments  of  the  same  commission  that  the 
strength  both  of  iron  and  steel,  subjected  to  strains  between 
the  extremes  of  temperature  of  boiling  water  and  freezing 
mercury,  was  greater  during  low  than  at  ordinary  tempera- 
tures. 

The  cheaper  methods  which  have  been  introduced  into  the 
manufacture  of  steel  within  but  a  few  years  past,  have  brought 
this  material  within  the  class  of  the  ordinary  materials  for 
engineering  purposes  ;  as  railroad  bars,  bridges,  etc.  ;  and  has 
led  to  a  very  extensive  series  of  experiments  upon  its  resist- 
ance to  the  usual  strains  on  building  materials;  among  the 
most  noted  of  which  are  those  of  Mr.  Fairbairn  and  of  Mr. 
Kirkaldy. 

The  results  of  Mr.  Fairbairn's  experiments,  Iteport  of  the 
British  Association,  1867,  give  for  the  mean  rupturing  strain 
from  extension  106,848  lbs.  per  square  inch  ;  and  for  com- 
pression a  mean  rupturing  strain  of  225,568  lbs.  per  square 
inch. 

From  the  same  series  of  experiments  upon  bars  deflected 
under  moderate  transverse  strains  the  coefficient  or  modulus 
of  elasticity  deduced  Avas  31,000,000  lbs.  per  square  inch. 

From  the  experiments  already  referred  to  by  Mr.  Kirkaldy, 
the  following  general  conclusions  were  arrived  at:  — 

1.  The  breaking  strain  of  steel,  when  taken  alone,  gives  no 
clue  to  the  real  qualities  of  various  kinds  of  that  metal  (74). 

2.  The  contraction  of  area  at  fracture  of  specimens  of  steel 
must  be  ascertained  as  well  as  in  those  of  iron  (74). 

3.  The  breaking  strain,  jointhj  with  the  contraction  of 
area,  affords  the  means  of  comparing  the  peculiarity  in  various 
lots  of  specimens  (74,  75). 

4.  Some  descriptions  of  steel  are  found  to  be  very  hard, 


BTKENGTH  OF  STEEL. 


165 


and,  consequently,  suitable  for  some  purposes,  whilst  others 
are  extremely  soft,  and  equally  suitable  for  other  uses  (74, 
75,  78). 

6.  The  breaking  strain  and  contraction  of  area  oijpuddled 
steel  plates,  as  in  iron  plates,  are  greater  in  the  direction  in 
which  they  are  rolled,  whereas  in  cast  steel  they  are  less  (74, 
75). 

6.  Steel  invariably  presents,  when  fractured  slowly,  a  silky 
fibrous  appearance  ;  when  fractured  suddenly  the  appearance 
is  invariably  granular,  in  which  case  the  fracture  is  always  at 
right"  angles  to  the  length ;  when  the  fracture  is  fibrous,  the 
angle  diverges  always  more  or  less  from  90°  (139). 

7.  The  granular  appearance  presented  by  steel  suddenly. 

8.  Steel  which  previously  broke  with  a  silky  fibrous  ap- 
pearance is  changed  into  granular  by  being  hardened  (141). 

9.  Steel  is  reduced  in  strength  by  being  hardened  in  water, 
while  the  strength  is  vastly  increased  by  being  hardened  in 
oil  (161,  1G2,  164). 

10.  The  higher  steel  is  heated  (without,  of  course,  running 
the  risk  of  being  burned)  the  greater  is  the  increase  of  strength, . 
by  being  plunged  into  oil  (161,  162). 

11.  In  a  highly  converted  or  hard  steel  the  increase  in 
strength  and  in  hardness  is  greater  than  in  a  less  converted  or 
soft  steel  (161,  162). 

12.  Heated  steel,  by  being  plunged  into  oil  instead  of 
water,  is  not  only  considerably  hardened,  but  toughened  by 
the  treatment  (162). 

13.  Steel  .plates  hardened  in  oil  and  joined  together  with 
rivets  are  fully  equal  in  strength  to  an  un jointed  soft  plate, 
or  the  loss  of  strength  by  riveting  is  more  than  counter- 
balanced by  the  increase  in  strength  by  hardening  in  oil 
(163). 

14.  Steel  rivets  fully  larger  in  diameter  than  those  used  in 
riveting  iron  plates  of  the  same  thickness  being  found  to  be 
greatly  too  small  for  riveting  steel  plates,  the  probability  is 
suggested  that  the  proper  proportion  for  iron  rivets  is  not,  as 
generally  assumed,  a  diameter  equal  to  the  thickness  of  the 
two  plates  to  be  joined  (163). 

15.  The  shearing  strain  of  steel  rivets  is  found  to  be  about 
a  fourth  less  than  the  tensile  strain  (163). 

16.  The  welding  of  steel  bars,  owing  to  their  being  so 
easily  burned  by  slightly  overheating,  is  a  difiicult  and  uncer- 
tain operation  (181,  15). 

17.  The  most  highly  converted  steel  does  not,  as  some  may 
suppose,  possess  the  greatest  density  (196). 


.  166 


CIVIL  ENGINEEErETG. 


18.  In  cast  steel  the  density  is  much  greater  than  in  pnd- 
died  steel,  which  is  even  less  than  in  some  of  the  superior  de- 
scriptions of  wrought  iron  (196). 

From  experiments  made  by  Major  "Wade,  late  of  the  XT.  S. 
Ordnance  Corps,  the  following  results  were  obtained  for  the 
crushing  weights  of  cast  iron  on  the  square  inch  : — 

Not  hardened  198,944  lbs. 

Hardened  ;  low  temper  354,544  " 

Hardened  ;  mean  temper  391,985  " 

Hardened;  high  temper  372,598  '* 

From  contracts  made  by  direction  of  Mr.  James  B.  Eads, 
chief  engineer  of  the  Illinois  and  St.  Louis  bridge,  at  St. 
Louis,  Missouri,  the  staves  of  the  arches,  the  pins  and  plates 
are  to  be  of  the  crucible  cast  steel  of  commerce.  Those  parts 
subjected  to  compression  are  to  withstand  60,000  pounds  on 
the  square  inch,  and  those  subjected  to  a  tensile  strain  40,000 
pounds  on  the  square  inch  without  permanent  set,  and  all 
must  stand  a  tensile  strain  of  100,000  pounds  on  the  square 
inch  without  fracture. 

The  modulus  of  elasticity  of  the  steel  not  to  be  less  than 
26,000,000  pounds,  nor  more  than  30,000,000. 


YIIL 

STRENGTH  OF  COPPER. 

The  various  uses  to  which  copper  is  applied  in  construc- 
tions, render  a  knowledge  of  its  resistance  under  various 
circumstaijces  a  matter  of  great  interest  to  the  engineer. 

376.  Resistance  to  Tensile  Strain.  The  resistance  of  cast 
copper  on  the  square  inch,  from  the  experiments  of  Mr.  G. 
Rennie,  is  8.51  tons,  that  of  wrought  copper  reduced  per 
hammer  at  15.08  tons.  Copper  wire  is  stated  to  bear  27.30 
tons  on  the  square  inch.  From  the  experiments  made  under 
the  direction  of  the  Franldin  Institute^  already  cited,  the 
mean  strength  of  rolled  sheet  copper  is  stated  at  14.35  tons 
per  square  inch. 

Besistance  to  Compressive  Strain.  Mr.  Rennie's  experi- 
ments on  cubes  of  one-fourth  of  an  inch  on  the  edge,  give  for 
the  crushing  weight  of  a  cube  of  cast  copper  7,318  lbs.,  and 
of  w^rought  copper  6,440  lbs. 


STRENGTH  OF  COPPER. 


167 


377.  Effects  of  Temperature  on  Tensile  Strength — . 

The  experiments  already  cited  of  the  Franklin  Institute, 
show  that  the  difference  in  strength  at  the  lower  tempera- 
tures, as  between  60°  and  90°,  is  scarcely  greater  than  what 
arises  from  irregularities  in  the  structure  of  the  metal  at 
ordinary  temperatures.  At  550°  Fahr.  copper  loses  one- 
fourth  of  its  tenacity  at  ordinary  temperatures,  at  817°  pre- 
cisely one-half^  and  at  1000°  two-thirds. 

Representing  the  results  of  experiments  by  a  curve  of 
which  the  ordinates  represent  the  temperatures  above  32°,  and 
the  abscissas  the  diminutions  of  tenacity  arising  from  increase 
of  temperature,  the  relations  between  the  two  will  be  thus 


378.  Mr.  Rennie  states  the  tenacity  of  cast  tin  at  2.11  tons 
per  square  inch ;  and  the  resistance  to  compression  of  a 
small  cube  of  \  of  an  inch  on  an  edge  at  966  lbs. 

In  the  same  experiments,  the  tenacity  of  cast  lead  is  stated 
at  0.81  tons  per  square  inch ;  and  the  resistance  of  a  small 
cube  of  same  size  as  in  preceding  paragraph  at  483  lbs. 

In  the  same  experirnents,  the  tenacity  of  hard  gun-metal  is 
stated  at  16.23  tons ;  that  of  fine  yellow  brass  at  8.01  tons. 
The  resistance  to  compression  of  a  cube  of  brass  the  same  as 
before  mentioned,  is  stated  at  10,301  lbs. 


IX. 


STRENGTH  OF  OTHER  METALS. 


X. 


LINEAR  CONTRACTION   AND   EXPANSION  OF  METALS   AND  OTHER 
MATERIALS  FROM  TEMPERATURE. 


379.  Coefficients  of  Linear  Expansion.— The  change  of 
length  which  takes  place  in  a  bar  of  any  material  estimated 
in  fractional  parts  of  its  length  at  0°  Centigrade,  for  a 


168 


CIVIL  ENGINEERINa. 


change  in  temperature  of  1°  Centigrade,  is  termed  the  coeffi- 
cient of  linear  exjoamion,  for  the  material  in  question. 

The  increase  in  length  for  other  changes  of  temperature 
than  1°  is  given  by  the  following  formula : — 

I  =  KNL, 

in  which  L  is  the  length  at  0°  C. ;  'N,  the  number  of  degrees 
above  0° ;  K,  the  coefficient  of  linear  expansion ;  and  I  the 
increase  of  length  due  to  N  degrees  above  0°  C. 


Table  of  Coefficients  of  Linear  Expansion  for  1°  C. 


DESCBIPTION  OF  MATERIAL. 


Authority. 


METALS. 

Antimony  , 

Bismuth  

Brass  (supposed  to  be  Hamburg-  plate  brass) 

"    (English  plate,  in  form  of  a  rod)  , 

"    (English  plate,  in  form  of  trough)  

*'  (cast)  

*'    (wire)  , 

Copper.  

Gold  (de  depart)  

"    (standard  of  Paris,  not  annealed)  

'*    (       "  "  annealed)  

Iron  (cast)  

"  (from  a  bar  cast  2  inches  square)  , 

"  (from  a  bar  cast  ^  an  inch  square). . . . , 

*'  (soft  forged)  , 

**  (round  wire)  

"  (wire)  

Lead  

Palladium  , 

Platina  

Silver  (of  cupel)  , 

*'     (Paris  standard)  , 

Solder  (white  ;  lead  2,  tin  1)  

*'      (spelter;  copper  2,  zinc  1)  

Speculum  metsJ  , 


Smeaton 

Eamsden 
u 

(( 

Smeaton 

Laplace  &  ) 
Lavoisier.  ] 


Ramsden 

Adie 
u 

Laplace  & 

Lavoisier 
(I 

Troughton 
Laplace  & 
Lavoisier 
Smeaton 
Wollaston 
Dulong  &  Petit 
Troughton 
Laplace  & 
Lavoisier 

Troughton 
Smeaton 


EXPANSION  DUE  TO  TEMPEHATUEE. 


169 


CEBCBIFTIOIT  OF  MATEBTAT.S. 


Steel  (untempered)  

**  (tempered  yellow,  annealed  at  65°  C). . 

*'  (blistered)  

(rod)  

Tin  (from  Malacca)  

"   (from  Falmoutli)  

Zinc  


TIMBEB. 

Baywood  (ia  tlie  direction  of  the  grain,  dry) . 


Deal  (in  the  direction  of  the  grain,  dry) 


STONE,  BRICK,  GLASS,  CEMENT. 


Arborath  pavement  

Brick  (best  stock)  

"  (fire)  

Caithness  pavement  

Cement  (Roman)  

Glass  (English  flint)  

"    (French  with  lead)  

Granite  (Aberdeen  gray)  ;  

' '     (Peterhead  red,  dry)  

"     (       "        "  moist)  

Greenstone  (from  Katho)  

Marble  (Carrara  moist)  

"       (    "  dry)  

"       (black  Gal way^  

"      (black,   softer  specimen,  containing 

"         more  fossils)  

"      (Sicilian,  white  moist)  

"       (     "        "  dry)  

Sandstone  (from  Craigleith  quarry)  

Slate  (from  Penrhyn  quarry,  Wales)  


Authority. 


Laplace  & ) 
Lavoisier  j" 


Smeaton 
Ramsden 
(  Laplace  &  ) 
(  Lavoisier  ) 


Smeaton 


Joule 


Adie 


Laplace  & 
Lavoisier 

Adie 


It  has  been  found  from  experiment  that  the  absorption  of 
water  in  any  manner  decreases  the  coefficient  of  linear  ex- 
pansion in  wood  ;  and  that,  in  some  cases,  in  stone  it  in- 
creases this  coefficient,  whilst  in  others  a  permanent  increase 
of  length  took  place  from  an  increase  of  temperature. 

An  increase  in  temperature  of  15°  C.  in  cast  iron,  and  8° 


170 


CIVIL  ENGINEEEING. 


C.  in  wrought  iron  will  produce  a  strain  of  one  ton  of  2240 
lbs.  on  the  square  inch,  when  the  two  ends  of  the  bar 
abut  against  a  Hxed  object. 


I- 

^  •  XI. 

ADHESION  OF  IRON  SPIKES  TO  TIMBER. 

380.  TiiE  following  tables  and  results  are  taken  from  an 
article  by  Professor  Walter  K.  Johnson,  published  in  the 
Journal  of  the  FranJdin  Institute,  Yol.  19,  1837,  giving  the 
details  of  experiments  made  by  him  on  spikes  of  various  forms 
driven  into  different  kinds  of  timber. 

The  first  series  of  experiments  was  made  with  Burden's 
plain  square  spike,  the  flanched,  grooved,  and  swell  spike,  and 
the  grooved  and  swelled  spike.  The  timber  was  seasoned 
Jersey  yellow  pine,  and  seasoned  white  oak. 

From  these  experiments  it  results,  that  the  grooved  and 
swelled  form  is  about  5  per  cent,  less  advantageous  than  the 
plain,  in  yellow  pine,  and  about  18 J  per  cent,  superior  to  the 
plain  in  oak.  The  advantage  of  seasoned  oak  over  the  sea- 
soned pine,  for  retaining  plain  spikes,  is  as  1  to  1.9,  and  for 
grooved  spikes  as  1  to  2.37. 

The  second  series  of  experiments,  in  wliich  the  timber  was 
soaked  in  water  after  the  spikes  were  driven,  gave  the  follow- 
ing results  : — ■ 

For  swelled  and  grooved  spikes,  the  order  of  retentiveness 
was :  1  locust ;  2  white  oak ;  3  hemlock ;  4  unseasoned  chest- 
nut ;  5  yellow  pine. 

For  grooved  spike  without  swell,  the  like  order  is :  1  un- 
seasoned chestnut ;  2  yellow  pine  ;  3  hemlock. 

The  swelled  and  grooved  spike  was,  in  all  cases,  found  to 
"be  inferior  to  the  same  spike  with  the  swell  filed  off. 

The  third  series  of  experiments  gave  the  following  results : 

Thoroughly  seasoned  oak  is  twice,  and  thoroughly  seasoned 
locust  2f  times  as  retentive  as  unseasoned  chestnut. 

The  forces  required  to  extract  spikes  are  more  nearly  pro- 
portional to  the  breadths  than  to  either  the  thickness  or  the 
weights  of  the  spikes.  And,  in  some  cases,  a  diminution  of 
thicKuess  with  the  same  breadth  of  spike  afforded  a  gain  in 
retentiveness. 


ADHESION  OF  lEON  SPIKES  TO  TTMBEE. 


171 


"  In  the  softer  and  more  spongy  kinds  of  wood  the  fibres, 
instead  of  being  forced  back  longitudinally  and  condensed 
npoii  themselves,  are,  by  driving  a  thick,  and  especially  a 
rather  obtusely-pointed  spike,  folded  in  masses  backward  and 
downward  so  as  to  leave,  in  certain  parts,  the  faces  of  the 
grain  of  the  timber  in  contact  with  the  surface  of  the 
metal." 

"  Hence  it  appears  to  be  necessary,  in  order  to  obtain  the 
greatest  effect,  that  the  fibres  of  the  wood  should  press  the 
faces  as  nearly  as  ]30ssible  in  their  longitudinal  direction,  and 
with  equal  intensities  throughout  the  whole  length  of  the 
spike." 

The  following  is  the  order  of  superiority  of  the  spikes  from 
that  of  the  ratio  of  their  weights  and  extracting  forces  respec- 
tively : — 

1.  Narrow  flat  

2.  Wide  flat  

3.  Grooved  but  not  swelled. 

4.  Grooved  and  not  notched. 

5.  Grooved  and  swelled, . .  . 

6.  Burden's  patent  

7.  Square  hammered  

8.  Plain  cylindrical  


7. 049  ratio  of  weight  to  extracting  force. 

5.712  "  "  " 

5.G62  "  "  " 

5.300  "  " 

4.624  "  " 

4.509  "  "  '* 

4.129  "  »  " 

3.200  *'  "  " 


"  All  the  experiments  prove,  that  when  a  spike  is  once 
started  the  force  required  for  its  final  extraction  is  much  less 
than  that  which  produced  the  first  movement." 

"  When  a  bar  of  iron  is  spiked  upon  wood,  if  the  spike  be 
driven  nntil  the  bar  compresses  the  wood  to  a  great  degree, 
the  recoil  of  the  latter  may  become  so  great  as  to  start  back 
the  spike  for  a  short  distance  after  the  last  blow  has  been 
given." 

342.  From  the  fourth  series  of  experiments  it  appears,  that 
the  spike  tapering  gradually  towards  the  cutting  edge  gives 
better  results  than  those  with  more  obtuse  ends. 

Tliat  beyond  a  certain  limit  the  ratio  of  the  weight  of  the 
spike  to  the  extracting  force  begins  to  diminish ;  showing 
that  it  would  be  more  economical  to  increase  the  number 
rather  than  the  length  of  the  spikes  for  producing  a  given 
eff(3ct." 

That  the  absolute  retaining  power  of  unseasoned  chestnnt 
on  square  or  flat  spikes  of  from  two  to  fonr  inches  in  length 
is  a  little  more  than  800  lbs.  for  every  square  inch  of  their 
two  faces  which  condense  longitudinally  the  fibres  of  the 
timber." 


CHAPTER  HI. 


MASONET. 

I.  Classificatiotn  of.  II.  Cut  Stone  Masonry.  III.  Rub- 
ble-Stone Masonky.  IY.  Bkick  Masonry.  Y.  Founda- 
tions OF  Structures  on  Land.  YI.  Foundations  of 
Structures  in  Water.    YII.  Construction  of  Masonry. 


SUMMARY. 

I. 

classification  of  masonry. 
Masonry  defined  and  classified  (Art.  381). 

n. 

CUT  STONE  MASONRY. 

Definitions  (Art.  383).     Requisites  of  Strength  (Arts.  384-390).  Bonds 
(Arts.  391-392).    Cutting  (Art.  393). 

III. 

RUBBLE-STONE  MASONRY. 
Quality  (Art.  394).    Construction  (Arts.  395-397). 

IY. 

brick  masonry. 


Construction  (Arts.  398-402).    Concrete  WaUs  (Arts.  403-416). 


MASONRY. 


173 


Y. 


FOUNDATIONS  OF  STKUCTFKES  ON  LAND. 


Foundation  defined  (Art.  417).  Importance  (Art.  418).  Classification  of 
Soils  (Art.  420).  Foundations  on  Rock  (Art.  421).  In  Stony  Ground 
(Arts.  422-423).  On  Sand  (Art.  424).  Precautions  against  water  (Art. 
425).  In  Compressible  Soils  (Arts.  426-429).  In  Marshy  Soils  (Art. 
480).  On  Piles  (Art.  431).  Pile  Engines  (Art.  432).  PHe  driving  (Arts. 
432-434).  Load  placed  on  piles  (Arts.  435-436).  Piles  prepared  for 
foundation  (Arts.  437-439).  On  Sand  (Art.  441).  Precautions  against 
Lateral  Yielding  (Art.  443). 


YI. 


FOUNDATIONS  OF  STEUOTUKES  IN  "WATEK. 


Difficulties  (Art.  444).  Use  of  Dams  (Arts.  445-449).  Use  of  Caisson  (Art. 
450).  Artificial  Island  (Art.  452).  Protection  against  running  water 
(Arts'.  454-455).  Pneumatic  processes  (Art.  456).  Pneumatic  piles 
(Arts.  457-458).    Pneumatic  Caissons  (Art.  459). 


YII. 


CONSTRUCTION  OF  MASONRY. 


Foundation  Courses  (Arts.  461-463).  Inverted  arches  (Art.  464).  Compo- 
nent parts  of  structures  of  Masonry  (Art.  467).  Walls  of  Enclosures 
(Art.  468).  Vertical  Supports  (Art.  469).  Areas  (Art.  470).  Retaining 
Walls  (Arts.  471-475).  Form  of  Section  of  Retaining  Walls  (Arts.  476- 
478).  Measures  for  increasing  the  Strength  of  Retaining  Walls  (Arts. 
479-488).  Counterforts  (Arts.  480-483).  Relieving  Arches  (Art.  484). 
Lintel  (Art.  490).  Plate-bande  (Art.  491).  Arches  (Arts.  492-494). 
Classification  of  Arches  (Art.  495).  Cylindrical  Arches  (Arts.  496-502). 
Oblique  Arch  (Arts.  502-503).  Groined  and  Cloistered  Arch  (Arts.  504- 
505).  Conical  Arch  (Art.  506).  Conoidal  Arch  (Arts.  507-508).  An- 
nular Arch  (Art.  509).  Dome  (Art.  510).  Arrangement  of  voussoirs 
(Arts.  511-513).  Construction  of  Arches  (Arts.  514-523).  Rupture  of 
Arches  (Arts.  524-527).  Precautions  to  be  observed  in  constructing 
Arches  (Arts.  528-533) ,  Precautions  against  settling  (Art.  534).  Point- 
ing (Arts.  535-537).  Repairs  of  Masonry  (Arts.  538-540).  EfEects  of 
Temperature  on  Masonry  (Art.  541). 


174 


CrVTL  ENGINEKRING. 


L 

CLASSIFICATION. 

381.  Masonry  is  the  art  of  raising  structures,  in  stone,  brick, 
and  mortar. 

Masonry  is  classified  either  from  the  nature  of  the  ma- 
terial, as  stone  masonry^  hricJc  masonry,  and  mixed,  or  that 
which  is  composed  of  stone  and  brick ;  or  from  the  manner  in 
which  the  material  is  prepared,  as  cut  stone  or  ashlar  masonry^ 
rubble-stone  or  rough  masonry^  and  hammered  stone  masonry  / 
or,  finally,  from  the  form  of  the  material,  d^^lregular  mason- 
ry, and  irregula/r  masonry. 


II. 

CUT  STONE. 

882.  Masonry  of  cut  stone,  when  carefully  made,  is  stronger 
and  more  solid  than  that  of  any  other  class  ;  but,  owing  to  the 
labor  required  in  dressing  or  preparing  the  stone,  it  is  also  the 
most  expensive.  It  is  therefore  mostly  restricted  to  those 
works  where  a  certain  architectural  effect  is  to  be  produced 
by  the  regularity  of  the  masses,  or  where  great  strength  is  in- 
dispensable. 

383.  Definitions.  Before  explaining  the  means  to  be  used 
to  obtain  the  greatest  strength  in  cut  stone,  it  will  be  neces- 
sary to  give  a  few  definitions  to  render  the  subject  clearer. 

In  a  wall  of  masonry  the  ievm  face  is  usually  applied  to  the 
front  of  the  wall,  and  the  term  hacJc  to  the  inside;  the  stone 
which  f(;rms  the  front,  is  termed  ih^fachig;  that  of  the  back, 
the  hacking ;  and  the  interior,  the  filling.  If  the  front,  or 
back  of  the  wall,  has  a  uniform  slope  from  the  top  to  the  bot- 
tom, this  slope  is  termed  the  hatter,  or  hatir. 

The  term  course  is  applied  to  each  horizontal  layer  of  stone 
in  the  wall :  if  the  stones  of  each  layer  are  of  equal  thickness 
throughout  it  is  termed  regular  coursing j  if  the  thicknesses 
are  unequal  the  term  random,  or  irregular  coursing,  is  ap- 
plied.   The  divisions  between  the  stones,  in  the  courses,  ai'e 


CrX-STONE  MASONRY. 


175 


termed  the  joints  ;  the  upper  surface  of  the  stones  of  each 
course  is  also  sometimes  termed  the  hed^  or  huild. 

The  arrangement  of  the  different  stones  of  each  course,  or 
of  contiguous  courses,  is  termed  the  bond. 

384.  Requisites  of  Strength.  The  strength  of  a  mass  of 
cut  stone  masonry  will  depend  on  the  size  of  the  blocks  in 
each  course,  on  the  accuracy  of  the  dressing,  and  on  the  bond 
used. 

The  size  of  the  blocks  varies  with  the  kind  of  stone  and  the 
nature  of  the  quarry.  From  some  quarries  the  stone  may  be 
obtained  of  any  required  dimensions ;  others,  owing  to  some 
peculiarity  in  the  formation  of  the  stone,  only  furnish  blocks 
of  small  size.  Again,  the  strength  of  some  stones  is  so  great 
as  to  admit  of  their  being  used  in  blocks  of  any  size,  without 
danger  to  the  stability  of  the  structure,  arising  from  their 
breaking ;  others  can  only  be  used  with  safety  when  the  length, 
breadth,  and  thickness  of  the  block  bear  certain  relations  to 
each  other.  No  fixed  rule  can  be  laid  down  on  this  point ; 
that  usually  followed  by  builders  is  to  make,  with  ordinary 
stone,  the  breadth  at  least  equal  to  the  thickness,  and  seldom 
greater  than  twice  this  dimension,  and  to  limit  the  length  to 
within  three  times  the  thickness.  When  the  breadth  or  the 
length  is  considerable,  in  comparison  with  the  thickness,  there 
is  danger  that  the  block  may  break,  if  any  unequal  settling, 
or  unequal  pressure  should  take  place.  As  to  the  absolute 
dimensions,  the  thickness  is  generally  not  less  than  one  foot, 
nor  greater  than  two  ;  stones  of  this  thickness,  with  the  rela- 
tive dimensions  just  laid  down,  will  weigh  from  1000  to  8000 
pounds,  allowing,  on  an  average,  160  pounds  to  the  cubic  foot. 
With  these  dimensions,  therefore,  the  weight  of  each  block 
will  require  a  very  considerable  power,  both  of  machinery  and 
men,  to  set  it  on  its  bed. 

385.  For  the  coping  and  top  courses  of  a  wall  the  same 
objections  do  not  apply  as  to  excess  in  length :  but  this  excess 
may,  on  the  contrary,  prove  favorable  ;  because  the  number 
of  top  joints  being  thus  diminished,  the  mass  beneath  the  co- 
ping will  be  better  protected,  being  exposed  only  at  the  joints, 
which  cannot  be  made  water-tight,  owing  to  the  mortar  being 
crushed  by  the  expansion  of  the  blocks  in  warm  weather,  and, 
when  they  contract,  being  washed  out  by  the  rain. 

386.  The  closeness  with  which  the  blocks  fit  is  solely  de- 
pendent on  the  accuracy  with  which  the  surfaces  in  contact 
are  wrought  or  dressed  ;  if  this  part  of  the  work  is  done  in  a 
slovenly  manner,  the  mass  will  not  only  present  open  joints 
from  any  inequality  in  the  settling  ;  but,  from  the  courses  not 


176 


CrVTL  ENGINEERING. 


fitting  accurately  on  their  beds,  the  blocks  will  be  liable  to 
crack  from  the  unequal  pressure  on  the  different  points  o£ 
the  block. 

387.  The  surfaces  of  one  set  of  joints  should,  as  a  prime 
condition,  be  perpendicular  to  the  direction  of  the  pressure : 
by  this  arrangement  there  will  be  no  tendency  in  any  of  the 
blocks  to  slip.  In  a  vertical  wall,  for  example,  the  pressure 
being  downward,  the  surfaces  of  one  set  of  joints,  which  are 
the  beds,  must  be  horizontal.  The  surfaces  of  the  other  set 
must  be  perpendicular  to  these,  and,  at  the  same  time,  perpen- 
dicular to  the  face,  or  to  the  back  of  the  wall,  according  to 
the  position  of  the  stones  in  the  mass ;  two  essential  points 
will  thus  be  attained, — the  angles  of  the  blocks,  at  the  top  and 
bottom  of  the  course,  and  at  the  face  or  back,  will  be  right 
angles,  and  the  block  will  therefore  be  as  strong  as  the  nature 
of  the  stone  will  admit.  The  principles  here  applied  to  a 
vertical  wall,  are  applicable  in  all  cases  whatever  may  be  the 
direction  of  the  pressure  and  the  form  of  the  exterior  sur- 
faces, whether  plane  or  curved. 

388.  A  modification  of  this  principle,  however,  may  in  some 
cases  be  requisite,  arising  from  the  strength  of  the  stone.  It 
is  laid  down  as  a  rule,  drawn  from  the  experience  of  builders, 
that  no  stone-work  with  angles  less  than  60°  will  offer  suffi- 
cient strength  and  durability  to  resist  accidents,  and  the  effects 
of  the  weather.  If,  therefore,  the  batter  of  a  wall  should  be 
greater  than  60°,  which  is  about  7  perpendicular  to  4  base, 
the  horizontal  joints  (Fig.  17)  must  not  be  carried  out  in  the 


Fig.  17— Represents  the  arrangement  of  stone  with 
abutting,  or  elbow  joints  for  very  incliucd  sur- 
faces. 
A,  face  of  the  block, 
c,  elbow  joint. 

■g         j        j         I  I         B,  buttress  block,  termed  a  newell  stone. 

same  plane  to  the  face  or  back,  but  be  broken  off  at  right 
angles  to  it,  so  as  to  form  a  small  abutting  joint  of  about  4 
inches  in  thickness.  As  the  batter  of  walls  is  seldom  so  great 
as  this,  except  in  some  cases  of  sustaining  walls  for  the  side 
slopes  of  earthen  embankments,  this  modilication  in  the  joints 
will  not  often  occur ;  for,  in  a  greater  batter,  it  will  generally 
be  more  economical,  and  the  construction  will  be  stronger,  to 
place  the  stones  of  the  exterior  in  offsets,  the  exterior  stone  of 
one  course  being  placed  within  the  exterior  one  of  the  course 
below'^it,  so  as  to  give  the  required  general  direction  of  the 


CTJT-STONE  MASONRY. 


177 


batter.  The  arrangement  with  offsets  has  the  further  advan- 
ta2^e  in  its  favor  of  not  allowing  the  rain  water  to  lodge  in  th© 
joint,  if  the  offset  be  slightly  bevelled  off. 

389.  Workmen,  unless  narrowly  watched,  seldom  take  the 
pains  necessary  to  dress  the  beds  and  joints  accurately  ;  on 
tlie  contrary,  to  obtain  what  are  termed  dose  joints,  they  dress 
the  joints  with  accuracy  a  few  inches  only  from  the  outward 
surface,  and  then  chip  away  the  stone  towards  the  back,  or 
tail  (Fig.  18),  so  that,  when  the  block  is  set,  it  will  be  in  con- 


Fig.  18 — Eepresents  a  section  of  a  wall  in  which  the 
face  is  of  cut  stone,  with  the  tails  of  the  blocks 
thinned  off,  and  the  backing  of  rubble. 

A,  section  of  face  block. 

B,  rubble  backing. 


tact  with  the  adjacent  stones  only  throughout  this  very  small 
extent  of  bearing  surface.  This  practice  is  objectionable 
under  every  point  of  view ;  for,  in  the  first  place,  it  gives  an; 
extent  of  bearing  surface,  which,  being  generally  inadequate- 
to  resist  the  pressure  thrown  on  it,  causes  the  ])lock  to  splinter 
off  at  the  joint ;  and  in  the  second  place,  to  give  the  block  its 
proper  set,  it  has  to  be  propped  beneath  by  small  bits  of  stone,, 
or  wooden  wedges,  an  operation  termed  pinning-up,  or  under- 
pinning, and  these  props,  causing  the  pressure  on  the  block 
to  be  thrown  on  a  few  points  of  the  lower  surface,  instead  of 
being  equally  diffused  over  it,  expose  the  stone  to  crack. 

390.  When  the  facing  is  of  cut  stone,  and  the  backing  of 
rubble,  the  method  of  thinning  off  the  block  may  be  allowed 
for  the  purpose  of  forming  a  better  bond  between  the  rubble 
and  ashlar  ;  but,  even  in  this  case,  the  block  should  be  dress- 
ed true  on  each  joint,  to  at  least  one  foot  back  from  the  face. 
If  there  exists  any  cause  which  would  give  a  tendency  to  an 
outward  thrust  from  the  back,  then  instead  of  thinning  off 
all  the  blocks  towards  the  tail  it  will  be  preferable  to  leave 
the  tails  of  some  thicker  than  the  parts  which  are  dressed. 

391.  Yarious  methods  are  used  by  builders  for  the  bond  of 
cut  stone.  The  system  termed  headers  and  stretchers,  in-i 
which  the  vertical  joints  of  the  blocks  of  each  course  alter- 

12 


178 


CIVIL  ENGESTEEEING. 


nate  with  the  vertical  joints  of  the  courses  above  and  below 
it,  or,  as  it  is  termed,  hreak  joints  with  them,  is  the  most  sim- 
ple, and  offers,  in  most  cases,  all  requisite  solidity.    In  this 


c 


Fig.  19  is  a  vertical  section  of  the  sea  walls  used  for  pro- 
tecting the  bhiif  s  of  the  islands  in  Boston  Harbor  ex- 
posed to  the  action  of  the  waves. 

A,  Stone  facing  of  heavy  blocks  well  fitted  and  clamped. 

B,  Concrete  bed  and  backing. 

C,  Top  wall  well  bonded. 

D,  Natural  soil  back  of  concrete. 


system  (Tig.  20),  the  blocks  of  each  course  are  laid  alter- 
nately with  their  greatest  and  least  dimensions  to  the  face  of 
the  wall ;  those  which  present  the  longest  dimension  along 
the  face  are  termed  stretchers ;  the  others,  headers.    If  the 


A 


n 


rj 


B 


Fig  20— Represents  an  elevation  A.  end  vie  w 
B,  and  plan  C,  of  a  wall  arranged  as  headers 
and  stretchers. 

a,  stretchers. 

6,  headers. 


header  reaches  from  the  face  to  the  back  of  the  wall,  it  is 
termed  a  through  ;  if  it  only  reaches  part  of  the  distance  it 
is  termed  a  hinder.  The  vertical  joints  of  one  course  are 
either  just  over  the  middle  of  the  blocks  of  the  next  course 
below,  or  else,  at  least  four  inches  on  one  side  or  the  other  of 
the  vertical  joints  of  that  course ;  and  the  headers  of  one 
coui-se  rest  as  nearly  as  practicable  on  the  middle  of  the 


CXJT-STONE  MASONRY. 


179 


stretchers  of  the  course  beneath.  If  the  backing  is  of  rubble, 
and  the  facing  of  cut  stone,  a  system  of  throughs  or  binders, 
similar  to  what  has  just  been  explained,  must  be  used. 

By  the  arrangement  here  described,  the  facing  and  backing 
of  each  course  are  well  connected ;  and,  if  any  unequal  set- 
tling takes  place,  the  vertical  joints  cannot  open,  as  would  be 
the  case  were  they  in  a  continued  line  from  the  top  to  the 
bottom  of  the  mass ;  as  each  block  of  one  course  confines  the 
ends  of  the  two  blocks  on  which  it  rests  in  the  course 
beneath. 

392.  In  masses  of  cut  stone  exposed  to  violent  shocks,  as 
those  of  which  light-houses,  and  sea-walls  in  very  exposed 
positions  are  formed,  the  blocks  of  each  course  require  to  be 
not  only  very  firmly  united  with  each  other,  but  also  with  the 
courses  above  and  below  them.  To  effect  this,  various  means 
have  been  used.  The  beds  of  one  course  are  sometimes  ar- 
ranged with  projections  (Fig.  21)  which  fit  into  correspond- 
ing indentations  of  the  next  course.  Iron  cramps  in  the  form 
of  the  letter  S,  or  in  any  other  shape  that  will  answer  the 


V 


Fig.  21 — Represents 
an  elevation,  A, 
plan,  B,  and  per- 
spective views,  G 
and  D,  of  two  of 
the  blocks  of  a  waU 
in  which  the  blocks 
are  fitted  with  in- 
dents, and  connect- 
ed with  bolts  and 
cramps  of  metal. 


purpose  of  giving  them  a  firm  hold  on  the  blocks,  are  let  into 
the  top  of  two  blocks  of  the  same  course  at  a  vertical  joint, 
and  are  firmly  set  with  melted  lead,  or  with  bolts,  so  as  to 
confine  the  two  blocks  together.  Holes  are,  in  some  cases, 
drilled  through  several  courses,  and  the  blocks  of  these 
courses  are  connected  by  strong  iron  bolts  fitted  to  the  holes. 

The  most  noted  examples  of  these  methods  of  strengthen- 
ing the  bond  of  cut  stone,  are  to  be  found  in  the  works  of  the 
Romans  which  have  been  preserved  to  our  time,  and  in  two 
celebrated  modern  structures,  the  Eddy-stone  and  Bell-rock 
light-houses  in  Great  Britain  (Fig.  22). 


180 


CIVIL  ENGINEERING. 


Fig.  22 — Kepresents  the  manner  of  arranging  stones  of 

the  same  course  by  dove-tail  joints  and  joggling,  taken 
from  a  horizontal  section  of  the  masonry  of  the  Bell- 
rock  light-house. 


Figs.  23,  24,  25,  26. — Plans  and  sections  showing  the 
masonry  bond  and  metal  fastenings  of  some  of  the  courses  in 
the  Minot's  ledge  light-house. 


Fig.  23.— Rock  surface  prepared  for  receiving  foundation. 


393.  The  manner  of  dressing  stone  belongs  to  the  stone- 
cutter's art,  but  the  engineer  should  not  be  inattentive  either 
to  the  accuracy  with  which  the  dressing  is  performed,  or  the 
means  employed  to  effect  it.  The  tools  chiefly  used  by  the 
workman  are  the  chisel,  axe,  and  hammer  for  hnotting.  The 
usual  manner  of  dressing  a  surface  is  to  cut  draughts  around 
and  across  the  stone  with  the  chisel,  and  then  to  use  the  chisel, 


CUT-STONE  MASONRY.  181 

the  axe  with  a  serrated  edge,  or  the  knotting  hammer,  to  work 
down  the  intermediate  portions  into  the  same  surface  with  the 
draughts.    In  performing  this  last  operation,  the  chisel  and 


Fig.  24. — Vertical  section  showing  foundation  courses  and  their  metal  fastenings. 


Fig.  26  —Vertical  section  and  interior  elevation  above  foundation  coursea. 


axe  should  alone  be  used  for  soft  stones,  as  the  grooves  on  the 
surface  of  the  hammer  are  liable  to  become  choked  bj  a  soft 


182 


CrVEL  ENGINEEEING. 


material,  and  the  stone  may  in  consequence  be  materially  in- 
jured by  the  repeated  blows  of  the  workman.  In  hard  stones 
this  need  not  be  apprehended. 

In  large  blocks  which  require  to  be  raised  by  machinery,  a  • 
hole,  of  the  shape  of  an  inverted  truncated  wedge,  is  cut  to 

pig.  27 — Kepresents  a  perspective 
view,  A,  of  a  block  of  stone  with 
draughts  around  the  edtres  of  its 
faces,  and  the  intermediate  space 
axed,  or  knotted,  and  its  tackling 
for  hoisting :    also  the  common 
iron  lewis,  B,  with  its  tackling, 
a,  draughts  around  edge  of  block. 
6,  knotted  part  between  draughts, 
c,  iron  bolts  with  eyes  let  into  oblique 

holes  cut  in  the  block. 
d  and  e,  chain  and  rope  tackling. 
n,  7i,  side  pieces  of  the  lewis. 
o,  centre  piece  of  lewis  with  eye  fast- 
ened to  71  n  by  a  bolt. 
p,  iron  ring  for  attaching  tackling. 


receive  a  small  iron  instrument  termed  a  lewis  (Fig.  27),  to 
which  the  rope  is  attached  for  suspending  the  block  ;  or  else, 
two  holes  are  cut  obliquely  into  the  block  to  receive  bolts 
with  eyes  for  the  same  purpose. 

When  a  block  of  cut  stone  is  to  be  laid,  the  first  point  to  be 
attended  to  is  to  examine  the  dressing,  which  is  done  by 
placing  the  block  on  its  bed,  and  seeing  that  the  joints  fit 
close,  and  the  face  is  in  its  proper  plane.  If  it  be  found  that 
the  fit  is  not  accurate,  the  inaccuracies  are  marked  and  the 
requisite  changes  made.  The  bed  of  the  course  on  which 
the  block  is  to  be  laid  is  then  thoroughly  cleansed  from  dust, 
&c.,  and  well  moistened,  a  bed  of  thin  mortar  is  laid  evenly 
over  it,  and  tlie  block,  the  lower  surface  of  which  is  first 
cleansed  and  moistened,  is  laid  on  the  mortar-bed,  and  well 
settled  by  striking  it  with  a  wooden  mallet.  Wlien  the  block 
is  laid  against  another  of  the  same  course,  the  joint  between 
them  is  prepared  with  mortar  in  the  same  manner  as  the 
bed. 


BRICK  MASONRY. 


183 


III. 

RUBBLE-STONE  MASONRY. 

394.  With  good  mortar,  rubble  work,  when  carefully  exe- 
cuted, possesses  all  the  strength  and  durability  required  in 
structures  of  an  ordinary  character ;  and  it  is  much  less  ex- 
pensive than  cut  stone. 

395.  The  stone  used  for  this  work  should  be  prepared 
simply  by  knocking  off  all  the  sharp,  weak  angles  of  the 
block ;  it  is  then  cleansed  from  dust,  &c.,  and  moistened, 
before  placing  it  on  its  bed.  This  bed  is  prepared  by  spread- 
ing over  the  top  of  the  lower  course  an  ample  quantity  of 
good  ordinary-tempered  mortar,  into  which  the  stone  is  firmly 
embedded.  The  interstices  between  the  larger  masses  of  stone 
are  filled  in  by  thrusting  small  fragments,  or  chippings  of 
stone,  into  the  mortar.  Finally,  the  whole  course  may  be 
carefully  grouted  before  another  is  commenced,  in  order  to 
fill  up  any  voids  left  between  the  full  mortar  and  stone. 

396.  To  connect  the  parts  well  together,  and  to  strengthen 
the  weak  points,  throughs  or  binders  should  be  used  in  all  the 
courses ;  and  the  angles  sliould  be  constructed  of  cut  or  ham- 
mered stone.  In  heavy  walls  of  rubble  masonry,  the  precau- 
tion, moreover,  should  be  observed,  to  lay  the  stones  on  their 
quarry-hed  /  that  is,  to  give  them  the  same  position,  in  the 
mass  of  masonry,  that  they  had  in  the  quarry  ;  as  stone  is 
found  to  offer  more  resistance  to  pressure  in  a  direction  per- 
pendicular to  the  quarry-bed  than  in  any  other.  The  direc- 
tions of  the  lamina  in  stratified  stones  show  the  position  of  the 
quarry-bed. 

397.  Hammered  stone,  or  dressed  rubble,  is  stone  roughly 
fashioned  into  regular  masses  with  the  hammer.  The  same 
precautions  must  be  taken  in  laying  this  kind  of  masonry  as 
in  the  two  preceding. 


lY. 

BRICK  MASONRY. 

398.  With  good  brick  and  mortar,  this  masonry  offers  great 
strength  and  durability,  arising  from  the  strong  adhesion  be- 
tween the  mortar  and  brick. 


184 


CIVIL  ENGINEERING. 


399.  The  bond  used  in  brick-work  is  very  various,  depend- 
ing on  the  character  of  the  structure.  The  most  usual  kinds 
are  known  as  the  English  and  Flemish.  The  first  consists  in 
arranging  the  courses  alternately,  entirely  as  headers  or 
stretchers,  the  bricks  through  the  course  breaking  joints.  In 
the  second  the  bricks  are  laid  as  headers  and  stretchers  in 
each  course.  The  lirst  is  stated  to  give  a  stonger  bond  than 
the  last ;  the  bricks  of  which,  owing  to  the  difficulty  of  pre- 
venting continuous  joints,  either  in  the  same  or  different 
courses,  are  liable  to  separate,  causing  the  face  or  the  back  to 
bulge  outward.  The  Flemish  bond  presents  the  finer  archi- 
tectural appearance,  and  is  therefore  preferred  for  the  fronts 
of  edifices. 

400.  Timber  and  iron  have  both  been  used  to  strengthen 
the  bond  of  brick  masonry.  Among  the  most  remarkable  ex- 
amples of  their  uses  are  the  well,  faced  in  brick,  forming  an 
entrance  to  the  Thames  Tunnel,  the  celebrated  work  of  Mr. 
Brunei,  and  his  experimental  arch  of  brick,  a  description  of 
which  is  given  in  the  Civil  Engineer  and  Architect's  Journal^ 
No.  6,  vol.  1.  In  both  these  structures  Mr.  Brunei  used  pan- 
tile laths  and  hoop  iron,  in  the  interior  of  the  horizontal 
courses,  to  connect  two  contiguous  courses  throughout  their 
length.  The  efficacy  of  this  method  has  been  further  fully 
tested  by  Mr.  Brunei,  in  experiments  made  on  the  resistance 
to  a  transversal  strain  of  a  brick  beam  bonded  with  hoop  iron, 
accounts  of  which,  and  of  experiments  of  a  like  kind,  are 
given  by  Colonel  Pasley  in  his  work  on  Limes,  Calcareom 
Cements,  &c. 

401.  The  mortar-bed  of  brick  may  be  either  of  ordinary  or 
thin-tempered  mortar;  the  last,  however,  is  the  best,  as  it 
makes  closer  joints,  and,  containing  more  water,  does  not  dry 
so  rapidly  as  the  other.  As  brick  has  great  avidity  for  water, 
it  would  always  be  well  not  only  to  moisten  it  befoi*e  laying 
it,  but  to  alloAV  it  to  soak  in  water  several  hours  before  it  is 
used.  By  taking  this  precaution,  the  mortar  between  the 
joints  will  set  more  firmly  than  when  it  imparts  its  water  to 
the  dry  brick,  which  it  frequently  does  so  rapidly  as  to  render 
the  mortar  pulvei'ulent  when  it  has  dried. 

402.  this  point  the  late  General  Totten,  Chief  of  Engin- 
eers, in  his  instructions  for  building  brick  masonry,  observes : 
"  The  want  of  cohesion  "  between  the  brick  and  mortar,  in  the 
case  of  some  gun  practice  against  brick  embrasures,  "  was 
due  to  the  interposition  of  dust,  sometimes  quite  free,  but 
more  generally  composing  a  layer  slightly  cohering  to  the 
body  of  the  bricks.    The  process  of  laying  must  be  to  cause 


BRICK  MASONEY.  185 

every  brick  to  be  thoroughly  soaked  in  water,  and  to  be  laid 
the  moment  it  ceases  to  drip." 

403.  Concrete  Walls.  The  use  of  hydraulic  concrete  for 
the  construction  of  both  solid  and  hollow  walls  for  houses  has 
very  much  increased  within  a  few  years ;  and  it  is  claimed 
that  they  are  drier,  stronger,  and  cheaper  than  walls  of  brick 
of  equal  thickness. 

In  some  of  the  cheaper  structures  of  this  class  put  up  in 
Paris,  the  concrete  was  composed  of  one  part  in  volume  of 
Portland  cement,  and  from  five  to  eight  parts  of  clean  screen- 
ed gravel  from  the  size  of  pearl  barley  to  that  of  peas ;  and  in 
some  cases  instead  of  gravel  what  is  known  as  brick  ballast, 
or  the  small  fragments  of  ordinary  brick  from  which  all  the 
fine  dust  is  screened  out,  is  used,  taking  eight  parts  of  this  to 
one  of  Portland  cement. 

404.  For  building  walls  of  concrete  where  a  scaffold  is  not 
necessary  it  is  only  requisite  to  have  a  boxing  formed  of 
scantling  and  boards  of  the  width  of  the  wall  within,  between 
the  two  sides  of  which  the  concrete  is  thrown  in  and  rammed. 

405.  For  solid  walls  requiring  a  scaffolding,  what  is  termed 
Tail's  bracket  scaffolding  is  used.    The  concrete  is  laid  with- 


Fig.  28  represents  a  vertical  section  of  the  boxing  for  laying 
concrete  walls. 

A,  Boarding  confined  by  clamp  screws. 

B,  Platform  supported  by  brackets  and  clamp  screws. 

C,  CyUnder  for  forming  flues  in  the  wall. 


in  the  boxing,  which  consist  of  boards.  A,  held  together  by 
clamp  screws,  h,  which  pass  through  hollow  iron  cones  placed 
between  the  sides  of  the  boxing,  which,  within,  is  of  the  same 
height  and  width  as  the  layer  of  concrete  to  be  laid  at  a  time. 
When  the  layer  is  finished  the  boxing  is  taken  apart,  and  the 
holes  left  by  the  cones  when  removed  are  used  for  secur- 
ing the  brackets  of  the  scaffolding,  which  consists  of  triangu- 
lar frames,  B,  each  formed  of  a  vertical  pin,  a  horizontal 
beam  to  support  the  flooring,  and  an  inclined  strut  to  support 
the  outer  end  of  the  horizontal  beam.  The  flooring,  of  suflfi- 
cient  width  for  the  workmen,  projects  beyond  the  wall  on  each 
side,  and  the  two  parts  without  and  within  are  held  together 


« 


186 


CIVIL  ENGmEERING. 


by  clamp  screws  which  pass  through  the  holes.  "WTien  cylin- 
drical flues  are  to  be  left  within  the  body  of  the  wall,  a  cylin- 
der C,  with  a  handle  to  it,  of  the  requisite  diameter,  and  the 
length  of  the  thickness  of  the  layer,  is  placed  in  position,  and 
the  concrete  rammed  well  around  it.  When  a  new  layer  is 
to  be  laid  the  cylinder  is  drawn  up  from  the  one  finished. 

406.  For  constructing  either  solid  or  hollow  walls,  an  ap- 
paratus devised  by  Mr.  Clarke  of  l!sew  Haven,  Conn.,  termed 
Clarke's  adjustable  frame  for  concrete  building,  is  used.  This 


Fig.  29,  Vertical  section  of  boxing  for  hollow  walls  of 
concrete. 

A,  Boxing  confining  concrete. 

B,  Horiz(jntal  arm  supporting  the  pieces  C. 
D,  Vertical  support  of  B. 

cr,  Clamp  screws  confining  C,  C. 

6,  Board  used  for  forming  the  void  in  the  waU. 


consists  of  a  boxing  of  boards.  A,  for  laying  the  concrete 
which  is  held  together  by  frames,  each  composed  of  a  hori- 
zontal piece,  B,  to  which  are  affixed  two  vertical  clamping 
pieces,  C,  the  interior  piece  being  movable  and  capable  of 
being  adjusted  by  screws,  the  two  pieces  being  held  together 
by  a  clamp  screw,  a  /  the  frames  and  boxing  being  attached 
to  vertical  supports,  D,  within  the  building,  in  which  holes 
are  arranged  at  suitable  distances  to  admit  of  the  frame  be- 
ing placed  at  the  proper  height.  For  hollow  Avails  a  wedge- 
shaped  board,  ^,  two  inches  and  a  half  thick  at  its  broad  end, 
and  two  inches  on  the  other,  is  used.  This  board  has  rect- 
angular notches  of  the  width  of  a  brick,  and  placed  at  twenty 
inches  apart,  cut  into  the  narrow  edge.  This  forms  the  core 
for  the  hollow  portion  of  the  wall.  The  work  is  started  or 
continued  by  placing  the  bricks  in  place  lengthwise  across  the 
hollow  so  as  to  tie  the  exterior  and  interior  portions  of  the 
wall  together.  The  core  is  then  placed  with  its  notches  fitting 
on  the  bricks,  and  secured  in  a  vertical  position,  the  concrete 
is  filled  in  on  each  side  between  the  sides  of  the  boxing. 
When  the  layer  is  finished  the  core  is  drawn  up. 


BEICK  MASONEY. 


187 


For  further  applications  of  Coignet  Beton,  see  Prof.  Bar- 
nard''s  Bejport  on  the  Paris  Exposition  0/  1867,  and  Gen. 
Gilmore^s  Pajper,  No.  19,  on  Beton  Agglomere. 

407.  Uses  of  beton  agglomere  in  Europe  and  else- 
where. The  most  important  and  costly  work  that  has  yet 
been  undertaken  in  this  material  is  a  section,  thirty-seven 
miles  in  length,  of  the  Yanne  aqueduct,  for  supplying  water 
to  the  city  of  Paris. 

This  aqueduct,  which  traverses  the  forest  of  Fontainebleau 
through  its  entire  length,  comprises  two  and  a  half  to  three 
miles  of  arches,  some  of  them  as  much  as  fifty  feet  in  height, 
and  eleven  miles  of  tunnels,  nearly  all  constructed  of  the  mate- 
rial excavated,  the  impalpable  sand  of  marine  formation 
known  under  the  generic  name  of  Fontainebleau  sand.  It  in- 
cludes, also,  eight  or  ten  bridges  of  large  span  (seventy-five  to 
one  hundred  and  twenty-five  feet)  for  the  bridging  of  rivers, 
canals,  and  highways. 

The  smaller  arches  are  full  centre,  and  are  generally  of  a 
uniform  span  of  ^^y^-q  feet,  with  a  thickness  at  the  crown  of 
15f  inches.  Their  construction  was  carried  on  without  inter- 
ruption through  the  winter  of  1868-'69  and  the  following 
summer,  and  the  character  of  the  work  was  not  affected  by 
either  extreme  of  temperature.  The  spandrels  are  carried 
up  in  open  work  to  the  level  of  the  crown,  and  upon  the 
arcade  thus  prepared  the  aqueduct  pipe  is  nioulded  in  the 
same  material,  the  whole  becoming  firmly  knit  together  into 
a  perfect  monolith.  The  pipe  is  circular,  6-J  feet  in  interior 
diameter,  with  a  thickness  of  9  inches  at  the  top,  and  12 
inches  at  the  sides,  at  the  water  surface.  The  construction  of 
the  arches  is  carried  on  about  two  weeks  in  advance  of  work 
on  the  pipe,  and  the  centres  are  struck  about  a  week  later. 

Water  was  let  into  a  portion  of  this  pipe  in  the  spring  of 
1869,  and  M.  Belgrand,  inspector-general  of  bridges  and 
highways,  and  director  of  drainage  and  sewers  of  the  city 
of  Paris,  certified  that  "  the  im/permsability  ajppeared  com- 
plete:' 

408.  Another  interesting  application  of  this  material  has 
been  made  in  the  construction,  completed  or  very  nearly  so,  of 
the  light-house  at  Port  Said,  Egypt.  It  will  be  one  hundred 
and  eighty  feet  high,  without  joints,  and  resting  upon  a  mon- 
olithic block  of  beton,  containing  nearly  four  hundred  cubic 
yards. 

409.  An  entire  Gothic  church,  with  its  foundations,  walls, 
and  steeple  in  a  single  piece,  has  been  built  of  this  material 
at  Yesinet,  near  Paris.    The  steeple  is  one  hundred  and 


188 


CIVIL  ENGINEERING. 


thirty  feet  high,  and  shows  no  cracks  or  other  evidences  of 
weakness. 

M.  FaUu,  the  founder,  certifies  that  "  during  the  two  years 
consumed  by  M.  Coignet  in  the  building  of  this  church,  the 
beton  agglomere,  in  all  its  stages,  was  exposed  to  rain  and 
frost,  and  that  it  has  perfectly  resisted  all  variations  of  tem- 
perature." 

The  entire  floor  of  the  church  is  paved  with  the  same  ma- 
terial, in  a  variety  of  beautiful  designs,  and  with  an  agreeable 
I       contrast  of  colors. 

410.  In  constructing  the  municipal  barracks  of  ISTotre 
Dame,  Paris,  the  arched  ceilings  of  the  cellars  were  made 
of  this  beton,  each  arch  being  a  single  mass.  The  spans 
varied  from  twenty-two  to  twenty-five  feet,  the  rise,  in 
in  all  cases,  being  one-tenth  the  span,  and  the  thickness  at 
the  crown  8.66  inches.  In  the  same  building  the  arched  ceil- 
ings of  the  three  stories  of  galleries,  one  above  the  other, 
facing  the  interior,  and  all  the  subterranean  drainage,  com- 
prising nearly  six  hundred  yards  of  sewers,  are  also  mono- 
liths of  beton. 

411.  Over  thirty-one  miles  of  the  Paris  sewers  had  been 
laid  in  this  material  prior  to  June,  1869,  at  a  saving  of  20  per 
cent.,  on  the  lowest  estimated  cost,  in  any  other  kind  of 
masonry. 

The  composition  of  the  beton  was  as  follows  : — 
Sand,  5  measures. 
Hydraulic  lime,  1  measure. 

Paris  cement  (said  to  be  as  good  as  Portland  cement),  ^ 
measure. 

412.  The  works  above  referred  to  were  ^dsited  by  the 
writer  in  the  month  of  February,  1870,  and  these  statements 
are  based  upon  close  observation  and  personal  knowledge. 

Many  other  interesting  applications  of  this  material  were 
examined,  of  which  it  is  not  deemed  necessary  to  make  any 
special  mention,  except  that  in  combined  stability,  strength, 
beauty,  and  cheapness  they  far  surpass  the  best  results  that 
could  have  been  achieved  by  the  use  of  any  other  materials, 
whether  stone,  brick,  or  wood. 

In  the  numerous  and  varied  applications  which  have  been 
made  of  it  in  France,  it  has  received  the  most  emphatic  com- 
mendations from  the  government  engineers  and  architects. 

413.  Its  superiority  to  Posendale  concrete  for  common 
work,  such  as  foundations,  the  backing  and  hearting  of  walls, 
magazine  walls,  and  generally  for  all  masonry  protected  by 
earth,  and  therefore  not  necessarily  required  to  be  of  first 


BEICK  MASONRY. 


189 


quality,  lies  in  its  possessing  greater  strength  and  hardness  at 
the  same  cost,  and  consequently  in  its  being  proportionately 
cheaper  when  reduced  to  the  same  strength  by  increasing 
the  proportion  of  sand. 

414.  Sea-water  is  nearly  as  good  as  fresh  water  for  mix- 
ing Portland  cements,  but  injures  the  Rosendale  and  all 
argillo-magnesian  cements  very  considerably. 

415.  It  is  of  great  importance  that  the  incorporation  of  the 
lime  with  the  cement  should  be  very  thorough,  in  order  to 
insure  a  perfectly  homogeneous  mixture,  and  this  can  be  ob- 
tained with  greater  certainty  by  triturating  the  two  together 
into  a  thick,  viscous  paste  before  the  sand  is  added.  In  con- 
ducting extensive  operations  the  use  of  two  mills  of  diiferent 
sizes  would  perhaps  be  advantageous,  the  smaller  one  being 
employed  exclusively  in  the  preparation  of  the  matrix. 

The  following  proportions  may  be  relied  upon  to  give 
Coignet  betons  of  good  average  quality : — 


1 

3 

3 

4 

6 
1 

1 

i 

7 
1 

f 

1 

416.  For  foundations  and  other  plain  massive  work  not  ex- 
posed to  view,  or  where  a  smooth  surface  is  not  specially  de- 
sired, a  liberal  amount  of  gravel  and  pebbles,  or  broken  stone, 
may  be  added  to  all  of  the  betons  of  the  above  table. 

The  following  proportions  will  answer  for  such  purposes : — 

1 

3 

3 

4 

6 
13 
1 

^■ 

6^ 
13 
1 

7 
13 
1 

f 

7i 
14 
1 

See  General  Gilmore^s  Beport, 


190 


CIVIL  ENGINEERING. 


Y. 

FOUNDATIONS  OF  STETICTTJEES  ON  LAND. 

417.  The  term  foundation  is  used  indifferently  either  for 
the  lower  courses  of  a  structure  of  masonry,  or  for  the  artifi- 
cial arrangement,  of  whatever  character  it  may  be,  on  which 
these  courses  rest.  For  more  perspicuity,  the  term  hed  of  the 
foundation  will  be  used  in  this  work  when  the  latter  is  de- 
signated. 

418.  The  strength  and  durability  of  structures  of  masonry 
depend  essentially  upon  the  bed  of  the  foundation.  In  ar- 
ranging this,  regard  must  be  had  not  only  to  the  permanent 
efforts  which  the  bed  may  have  to  support,  but  to  those  of  an 
accidental  character.  It  should,  in  all  cases,  be  placed  so  far 
below  the  surface  of  the  soil  on  which  it  rests,  that  it  will  not 
be  liable  to  be  uncovered,  or  exposed  ;  and  its  surface  should' 
not  only  be  normal  to  the  resultant  of  the  efforts  which  it  sus- 
tains, but  this  resultant  should  intersect  the  base  of  the  bed 
so  far  within  it,  that  the  portion  of  the  soil  between  this  point 
of  intersection  and  the  outward  edge  of  the  base  shall  be 
broad  enough  to  prevent  its  yielding  irom  the  pressure  thrown 
on  it. 

419.  The  first  preparatory  step  to  be  taken,  in  determining 
the  kind  of  bed  required,  is  to  ascertain  the  nature  of  the  sub- 
soil on  which  the  structure  is  to  be  raised.  This  may  be  done, 
in  ordinary  cases,  by  sinking  a  pit ;  but  where  the  subsoil  is 
composed  of  various  strata,  and  the  structure  demands  extra- 
ordinary precaution,  borings  must  be  made  with  the  tools 
usually  employed  for  this  purpose. 

420.  Classification  of  Soils. — With  respect  to  foundations, 
soils  are  usually  divided  into  three  classes : 

The  1st  class  consists  of  soils  which  are  incompressible,  or, 
at  least,  so  slightly  compressible,  as  not  to  affect  the  stability 
of  the  heaviest  masses  laid  upon  them,  and  which,  at  the  same 
time,  do  not  yield  in  a  lateral  direction.  Solid  rock,  some 
tufas,  compact  stony  soils,  hard  clay  which  yields  only  to  the 
pick  or  to  blasting,  belong  to  this  class. 

The  2d  class  consists  of  soils  which  are  incompressible,  but 
require  to  be  confined  laterally,  to  prevent  them  from  spread- 
ing out.    Pure  gravel  and  sand  belong  to  this  class. 

The  3d  class  consists  of  all  the  varieties  of  compressible 
soils  ;  under  which  head  may  be  arranged  ordinary  clay,  the 


FOUNDATIONS  OF  STKUCTTJEES. 


191 


common  earths,  and  marshy  soils.  Some  of  this  class  are 
found  in  a  more  or  less  compact  state,  and  are  compressible 
only  to  a  certain  extent,  as  most  of  the  varieties  of  clay  and 
common  earth  ;  others  are  found  in  an  almost  fluid  state,  and 
yield,  with  facilit}^,  in  every  direction. 

421.  Foundations  on  Rock. — To  prepare  the  bed  for  a 
foundation  on  rock,  the  thickness  of  the  stratum  of  rock 
should  first  be  ascertained,  if  there  are  any  doubts  respecting 
it :  and  if  there  is  any  reason  to  suppose  that  the  stratum  has 
not  sufficient  strength  to  bear  the  weight  of  the  structure,  it 
should  be  tested  by  a  trial  weight,  at  least  twice  as  great  as 
the  one  it  will  have  to  bear  permanently.  The  rock  is  next 
properly  prepared  to  receive  the  foundation  courses  by  level- 
ling its  surface,  which  is  effected  by  breaking  down  all  pro- 
jecting points,  and  filling  up  cavities,  either  with  rubble  ma- 
sonry or  with  beton ;  arid  by  carefully  removing  any  portions 
of  the  upper  stratum  which  present  indications  of  having  been 
injured  by  the  weather.  The  surface,  prepared  in  this  man- 
ner, should,  moreover,  be  perpendicular  to  the  direction  of  the 
pressure ;  if  this  is  vertical,  the  surface  should  be  horizontal, 
and  so  for  any  other  direction  of  the  pressure.  Should  there, 
however,  be  any  difficulty  in  so  arranging  the  surface  as  to 
have  it  normal  to  the  resultant  of  the  pressure,  it  may  receive 
a  position  such  that  one  component  of  the  resultant  shall  be 
perpendicular  to  it,  and  the  other  parallel ;  'the  latter  being 
counteracted  by  the  friction  and  adhesion  between  the  base 
of  the  bed  and  the  surface  of  the  rock.  If,  owiiig  to  a  great 
declivity  of  the  surface,  the  whole  cannot  be  brought  to  the 
same  level,  the  rock  must  be  broken  into  steps,  in  order  that 
the  bottom  courses  of  the  foundation  throughout,  may  rest  on 
a  surface  perpendicular  to  the  direction  of  the  pressure.  If 
fissures  or  cavities  are  met  with,  of  so  great  an  extent  as  to 
render  the  filling  them  with  masonry  too  expensive,  an  arch 
must  then  be  formed,  resting  on  the  two  sides  of  the  fissure, 
to  support  that  part  of  the  structure  above  it. 

The  slaty  rocks  require  most  care  in  preparing  them  to  re- 
ceive a  foundation,  as  their  top  stratum  will  generally  be 
found  injured  to  a  greater  or  less  depth  by  the  action  of  frost. 

422.  Foundations  in  Stony  Ground.— In  stony  earths  and 
hard  clay,  the  bed  is  prepared  by  digging  a  trench  wide 
enough  to  receive  the  foundation,  and  deep  enough  to  reach 
the  compact  soil  which  has  not  been  injured  by  the  action  of 
frost ;  a  trench  from  4  to  6  feet- will  generally  be  deep  enough 
for  this  purpose. 

423.  In  compact  gravel  and  sand,  where  there  is  no  lia- 


192 


CIVIL  ENGINEERING. 


bility  to  lateral  yielding,  either  from  the  action  of  rain  or  any 
other  cause,  tlie  bed  may  be  prepared  as  in  the  case  of  stony 
earths.  If  tliere  is  danger  from  lateral  yielding,  the  part  on 
which  the  foundation  is  to  rest  must  be  secured  by  confining 
it  laterally  by  means  of  sheeting  piles,  or  in  any  other  way 
that  w^ill  offer  sufficient  security. 

424.  Foundations  on  Sand.— In  laying  foundations  on 
firm  sand,  a  further  precaution  is  sometimes  resorted  to,  of 
placing  a  platform  on  the  bottom  of  the  trench,  for  the  pur- 
pose of  distributing  the  whole  weight  more  uniformly  over  it. 
This,  how^ever,  seems  to  be  unnecessary ;  for  if  the  bottom 
courses  of  the  masonry  are  well  settled  in  their  bed,  there  is 
no  good  reason  to  apprehend  any  unequal  settling  from  the 
effect  of  the  superincumbent  weight :  whereas,  if  the  wood  of 
the  platform  should,  by  any  accident,  give  way,  it  would  leave 
a  part  of  the  foundation  without  any  support. 

Wlien  the  sand  under  the  bed  is  liable  to  injury  from 
springs  they  must  be  cut  off,  and  a  platform,  or,  still  better, 
an  area  of  beton,  should  compose  the  bed,  and  this  should  be 
confined  on  all  sides  between  walls  of  stone,  or  beton  sunk 
below  the  bottom  of  the  bed. 

425.  Precautions  against  Water, — If,  in  opening  a  trench 
in  sand,  w^ater  is  found  at  a  slight  depth,  and  in  such  quan- 
tity as  to  impede  the  labors  of  the  workmen,  and  the  trench 
cannot  be  kept  -dry  by  the  use  of  pumps  or  scoops,  a  row  of 
sheeting  piles  must  be  driven  on  each  side  of  the  space  occu- 
pied by  it,  somewhat  below  the  bottom  of  the  bed,  the  sand 
on  the  outside  of  the  sheeting  piles  be  thrown  out,  and  its 
place  filled  with  a  puddling  of  clay,  to  form  a  water-tight  en- 
closure round  the  trench.  The  excavation  for  the  bed  is  then 
commenced  ;  but  if  it  be  found  that  tlie  water  still  makes 
rapidly  at  the  bottom,  only  a  small  portion  of  the  trench  must 
be  opened,  and  after  the  lower  courses  are  laid  in  this  por- 
tion, the  excavation  will  be  gradually  effected,  as  fast  as  the 
workmen  can  execute  the  work,  without  difficulty  from  the 
water. 

426.  Foundations  in  Compressible  Soils.  The  beds  of 
foundations  in  compressible  soils  require  peculiar  care,  parti- 
cularly when  the  soil  is  not  homogeneous,  presenting  more 
resistance  to  pressure  in  one  point  than  in  another ;  for,  in 
that  case,  it  will  be  very  difficult  to  guard  against  unequal 
settling. 

427.  In  ordinary  clay,  or  earth,  a  trench  is  dug  of  the  pro- 
per width,  and  deep  enough  to  reach  a  stratum  beyond  the 
action  of  frost ;  the  bottom  of  the  trench  is  then  levelled  off 


FOTJNDATIONS  OF  STKTJCTrEES. 


193 


to  receive  the  foundation.  This  may  be  laid  immediately  on 
the  bottom,  or  else  upon  a  grillage  and  ^platform.  In  the 
first  case,  the  stones  forming  the  lowest  course  should  be 
firmly  settled  in  their  beds,  by  ramming  them  with  a  very 
heavy  beetle.  In  tlie  second  a  timber  grating,  termed  a  gril- 
lage (Fig.  30),  w^hich  is  formed  of  a  course  of  heavy  beams 
laid  lengtlnvise  in  the  trench,  and  connected  firmly  by  cross 
pieces  into  which  they  are  notched,  is  firmly  settled  in  the 
bed,  and  the  earth  is  solidly  packed  between  the  longitudinal 
and  cross  pieces ;  a  flooring  of  thick  planks,  termed  a  plat- 
form, is  then  laid  on  the  grillage,  to  receive  the  lowest  course 


Fig.  80  represents  the  arrangement  of  a  grillage  and  platf orn) 

fitted  on  piles. 
A,  masonry, 
aa,  piles. 
&,  string-pieces. 

c,  cross  pieces. 

d,  capping-piece. 

e,  platform  of  plank. 


of  the  foundation.  The  object  of  the  grillage  and  platform, 
is  to  diffuse  the  weight  more  uniformly  over  the  surface  of 
the  trench,  to  prevent  any  part  from  yielding. 

428.  Repeated  failures  in  grillages  and  platforms,  arising 
either  from  the  compression  of  the  woody  fibre  or  from  a 
transversal  strain  occasioned  by  the  subsoil  offering  an  unequal 
resistance,  have  impaired  confidence  in  their  efiicacy.  En- 
gineers now  prefer  beds  formed  of  an  area  of  beton,  as  offer- 
ing more  security  than  any  bed  of  timber,  either  in  a  ami- 
formly  or  unequally  compressible  soil. 

429.  The  preparation  of  an  area  of  beton  for  the  bed  of  a 
foundation,  will  depend  on  the  circumstances  of  the  case.  In 
ordinary  cases  the  beton  is  thrown  into  the  trench,  and  care- 
fully rammed  in  layers  of  6  or  9  inches,  until  the  mortar  col- 
lects in  a  semi-fluid  state  on  the  top  of  the  layer.  If  the 
base  of  the  bed  is  to  be  broader  than  the  top,  its  sides  must 
be  confined  by  boards  suitably  arranged  for  this  purpose. 
Whenever  a  la^^er  is  left  incomplete  at  one  end,  and  another 
is  laid  upon  it,  an  offset  should  be  left  at  the  unfinished  ex- 

13 


194 


CIVIL  ENGINEEETNG. 


tremity,  for  the  purpose  of  connecting  tlie  two  layers  more 
fii-mly  when  the  work  on  the  nnlinished  part  is  resumed. 

Considerable  economy  may  be  effected,  in  the  quantity  of 
bdton  required  for  the  bed,  by  using  large  blocks  of  stone 
which  should  be  so  distributed  throughout  the  layer  that  the 
beetle  will  meet  with  no  difficulty  in  settling  the  beton  be- 
tween and  around  the  blocks. 

When  springs  rise  through  the  soil  over  which  the  beton  is 
to  be  spread,  the  water  from  them  must  either  be  conveyed  off 
by  artificial  channels,  which  will  prevent  it  rising  through  the 
mass  of  beton  and  washing  out  the  lime ;  or  else  strong  cloth, 
prepared  so  as  to  be  impermeable  to  water,  may  be  laid  over 
the  surface  of  the  soil  to  receive  the  bed  of  beton. 

The  artificial  channels  for  conveying  off  the  water  may  be 
formed  either  of  stone  blocks  with  semi-cylindrical  channels 
cut  in  them,  or  of  semi-cylinders  of  iron,  or  tiles,  as  may  be 
most  convenient.  A  sufficient  number  of  these  channels 
should  be  formed  to  give  an  outlet  to  the  water  as  fast  as  it 
rises. 

An  impermeable  cloth  may  be  formed  of  stout  canvas, 
prepared  with  bituminous  pitch  and  a  drying  oil.  It  is  well 
to  have  the  cloth  doubled  on  each  side  with  ordinary  canvas 
to  prevent  accidents.  The  manner  of  settling  the  cloth  on 
the  surface  of  the  soil  must  depend  on  the  circumstances  of 
the  case. 

Each  of  the  foregoing  expedients  for  preventing  the  action 
of  springs  on  an  area  of  beton  has  been  tried  with  success. 
When  artificial  channels  are  used,  they  may  be  completely 
choked  subsequently,  by  injecting  into  them  a  semi-fluid 
hydraulic  cement,  and  the  action  of  the  springs  be  thus  de- 
stroyed. 

Foundation  beds  of  beton  are  frequently  made  without  ex- 
hausting the  water  from  the  area  on  which  they  are  laid.  For 
this  purpose  the  beton  is  thrown  in  layers  over  the  area,  by 
using  either  a  wooden  conduit  reaching  nearly  to  the  position 
of  the  layer,  or  else  by  placing  the  beton  (Fig.  31)  in  a  box 
by  which  it  is  lowered  to  the  position  of  the  layer,  and  from 
which  it  is  deposited  so  as  not  to  permit  the  water  to  separate 
the  lime  from  the  other  ingredients. 

A  conduit  for  immersing  hydraulic  concrete,  formed  of 
boiler  iron,  has  been  used  on  some  of  our  public  works.  The 
body  of  it  is  cylindrical,  and  made  in  sections  which  can  be 
readily  successively  fastened  on  or  detached  ;  the  bottom, 
having  the  form  of  a  conical  frustum,  is  fastened  to  the  low- 
est section  of  the  body.    The  conduit  is  suspended  vertically 


FOUNDATIONS  OF  STRUCTUEE8.  195 


from  a  movable  crane,  or  crab  engine,  by  a  strong  screw,  by 
which  it  can  be  raised  or  lowered,  so  as  to  admit  the  concrete 
to  escape  from  the  body  through  the  conical-shaped  end,  to 
be  spread  and  compressed  by  the  movements  of  the  crane  and 
screw. 


Fig.  31  represents  an  end 
view,  A,  of  a  semi-cylindrical 
box  for  lowering  b^ton  in 
water,  and  B  the  same  view  of 
the  box  when  opened  to  let  the 
b6ton  fall  through. 

o,  hinge  around  which  the 
halves  of  the  box  open. 

a,  rope  tackling  for  lowering 
box. 

&,  pin,  or  catch  to  fasten  the 
two  parts  of  the  box. 

c,  cord  to  detach  the  pin  and 
open  the  box. 


Should  it  be  found  that  springs  boil  up  at  the  bottom,  it 
must  be  covered  with  an  impermeable  cloth. 

430.  Foundations  in  Marshy  Soils.  In  marshy  soils  the 
principal  difficulty  consists  in  forming  a  bed  sufficiently  firm 
to  give  stability  to  the  structure,  owing  to  the  yielding  nature 
of  the  soil  in  all  directions. 

The  following  are  some  of  the  dispositions  that  have  been 
tried  with  success  in  this  case.  Short  piles  from  6  to  12  feet 
long,  and  from  6  to  9  inches  in  diameter,  are  driven  into  the 
soil  as  close  together  as  they  can  be  crowded,  over  an  area 
considerably  greater  than  that  which  the  structure  is  to  occu- 
py. The  heads  of  the  piles  are  accurately  brought  to  a  level 
to  receive  a  grillage  and  platform ;  or  else  a  layer  of  clay, 
from  4  to  6  feet  thick,  is  laid  over  the  area  thus  prepared  with 
piles,  and  is  either  solidly  ram^med  in  layers  of  a  foot  thick, 
or  submitted  to  a  very  hea^^  pressure  for  some  time  before 
commencing  the  foundations.  The  object  of  preparing  the 
bed  in  this  manner  is  to  give  the  upper  stratum  of  the  soil  all 
the  firmness  possible,  by  subjecting  it  to  a  strong  compression 
from  the  piles  ;  and  when  this  has  been  effected,  to  procure  a 
firm  bed  for  the  lowest  course  of  tlie  foundation  by  the  gril- 
lage, or  clay  bed ;  by  these  means  the  whole  pressure  will  be 
uniformly  distributed  throughout  the  entire  area.  This  case 
is  also  one  in  which  a  bed  of  beton  would  replace,  with  great 
advantage,  either  the  one  of  clay,  or  the  grillage. 


I 

196 


CIVIL  ENGINEERING. 


The  purposes  to  which  the  short  piles  are  applied  in  this 
case  is  different  from  the  object  to  be  attained  usually  in  the 
emplo3^ment  of  piles  for  foundations  ;  which  is  to  transmit  the 
weight  of  the  structure  that  rests  on  the  piles,  to  a  Urm  in- 
compressible soil,  overlaid  by  a  compressible  one,  that  does 
not  offer  sufficient  firmness  for  the  bed  of  the  foundation. 

431.  Foundations  on  Piles.  When  a  firm  soil  is  overlaid 
by  one  of  a  compressible  character,  and  its  distance  below  the 
surface  is  such  that  it  can  be  reached  by  piles  of  ordinary  di- 
mensions, they  should  be  used  in  preference  to  any  other  plan, 
when  they  can  be  rendered  durable,  on  account  of  their 
economy  and  the  security  they  afford. 

To  prepare  the  bed  to  receive  the  foundations  in  this  case, 
strong  piles  are  driven,  at  equal  distances  apart,  over  the  en- 
tire area  on  which  the  structure  is  to  rest.  These  piles  are 
driven  until  they  meet  with  a  firm  stratum  below  the  com- 
pressible one,  which  offers  sufficient  resistance  to  prevent  them 
from  penetrating  farther. 

Piles  are  generally  from  9  to  18  inches  in  diameter,  with  a 
length  not  above  20  times  the  diameter,  in  order  that  they 
may  not  beud  under  the  stroke  of  the  ram.  They  are  pre- 
pared for  driving  by  stripping  them  of  their  bark,  and  paring 
down  the  knots,  so  that  the  friction,  in  driving,  may  be  re- 
duced as  much  as  possible.  The  head  of  the  pile  is  usually 
encircled  by  a  strong  hoop  of  wrought  iron,  to  prevent  the  pile 
from  being  split  by  the  action  of  the  ram.  The  foot  of  the 
pile  may  receive  a  shoe  formed  of  ordinary  boiler  iron,  well 
fitted  and  spiked  on ;  or  a  cast-iron  shoe  of  a  suitable  form 
for  penetrating  the  soil  may  be  cast  around  a  wrought-iron 
bolt,  by  means  of  which  it  is  fastened  to  the  pile. 


Figr.  32  represents  a  section  through  the  axis  of  a  cast-iron  shoe  and  wrought- 
iron  bolt  for  a  pile. 


432.  Screw  Piles.  In  localities  where  it  has  been  found  im- 
practicable to  resort  to  any  of  the  usual  means  of  foundations, 
as  on  sandpits,  or  on  beds  of  a  soft  conglomerate  formed 
of  shells,  clay,  and  the  oxide  of  iron,  such  as  are  found  on  our 
Southern  coasts,  iron  screw  piles  have  been  used  with  success, 
particularly  for  light-house  structures  of  iron. 


FOUNDATIONS  OF  STKUCTURES. 


197 


These  piles  have  the  screws  of  different  forms  according  to 
the  soil  they  are  to  be  used  in.  The  point  being  little  or 
nothing,  and  the  thread  of  the  screw  very  broad,  for  loose 


^^S.  34.  Fig, 


Figs.  33,  34,  35.  Elevations  of  screw  piles  for  loose,  firm  and  hard  or  rocky  soil  respectively. 
A,  newel ;  B,  thread  of  screw. 

soils ;  the  point  becoming  sharper  and  the  thread  of  the  screw 
more  narrow  as  the  soil  becomes  harder. 

Disk  Piles.  In  some  parts  of  India  this  species  of  pile  has 
been  advantageously  employed. 


A 

Fig.  36.   Elevation  of  a  disk  pile.   A,  shaft ;  B,  disc ;  C,  water-hole. 


These  piles  are  made  hollow  of  iron,  and  have  a  circular 
disk  attached  to  the  foot.  A  hole  is  made  in  the  disk  to 
allow  water  to  pass  through. 

Pile  Engines.  A  machine,  termed  2ipile  engine^  is  used 
for  driving  piles.  It  consists  essentially  of  two  upriglits 
firmly  connected  at  top  by  a  cross  piece,  and  of  a  mm,  or 
monkey  of  cast  iron,  for  driving  the  pile  by  a  force  of  per- 
cussion. Two  kinds  of  engines  are  in  use ;  the  one  termed  a 
crah  engine^  from  the  machinery  used  to  hoist  the  ram  to  the 
height  from  which  it  is  to  fall  on  the  pile  ;  the  other  the  ring- 
ing engine,  from  the  monkey  being  raised  by  the  sudden  pull 
of  several  men  upon  a  rope,  by  which  the  ram  is  drawn  up  a 
few  feet  to  descend  on  the  pile. 

The  crab  engine  is  by  far  the  more  powerful  machine,  but 
on  this  account  is  inapplicable  in  some  cases,  as  in  the  driving 


198 


CIVIL  ENGINEERING. 


of  cast-iron  piles,  where  the  force  of  the  blow  might  destroy 
the  pile ;  also  in  long  slender  piles  it  may  cause  the  pile  to 
spring  so  much  as  to  prevent  it  from  entering  the  subsoil. 

The  steam  jpile  driver  is  but  a  modification  of  the  Cfrah 
engine. 


Fig.  37  represents  a  front  elevation  of  the  gun- 
powder pile  driver. 
AA,  guides. 

B,  ram. 

C,  socket  in  which  piston  I  fits 

D,  cast-iron  cylinder   containing  powder 
chamber  E. 

F,  socket  to  fit  on  head  of  pile. 

G,  pile. 

K,  plunger  of  ram. 

L,  lever  to  hold  ram  at  any  point  on  the 
guides. 


Shaw's  gunpowder  pile  driver  consists  essentially  of  two 
uprights  or  guides,  between  which  are  placed  the  ram  and 
powder  chamber.  The  latter  consists  of  a  cast-iron  cylinder, 
having  a  socket  in  its  lower  end,  and  a  powder  chamber  at 
the  upper.  The  ram  differs  from  that  in  ordinary  use  only 
by  having  a  plunger  made  to  fit  the  powder  chamber,  at  the 
bottom,  and  a  cylindrical  cavity  at  top,  extending  about  half 
way  down.  At  any  convenient  point  on  the  guides  is  placed 
a  piston  made  to  fit  into  the  ram,  to  take  the  place  of  an  air- 
cushion  in  taking  up  the  recoil,  in  case  the  charge  should  be 


too  great. 


AYork  is  begun  by  placing  the  powder  chamber  on  top  of 


FOUNDATIONS  OF  STRUCTURES. 


199 


the  pile  to  be  driven,  putting  a  cartridge  in  the  chamber,  and 
allowing  the  ram  to  fall.  The  explosion  of  the  cartridge 
throws  the  ram  up  and  drives  the  pile  down  proportionally. 
Another  cartridge  is  thrown  in  and  the  operation  repeated. 
The  only  limit  to  the  rapidity  of  the  blows  is  the  size  of  the 
cartridges  and  the  rapidity  with  which  they  are  supplied. 


Pig.  38  represents  the  capstan  for  driving  screw  piles  by  hand. 

A,  shaft  of  pile. 

B,  screw. 

C,  capstan. 

D,  taper  of  shaft  to  fit  into  socket  of  next  section  above. 

£,  bolt  fastening  socket  of  shaft  to  taper  of  next  section  be* 
km. 


For  driving  screw-piles  a  capstan  is  fitted  to  the  head  of 
the  pile,  and  motion  communicated  to  the  pile  either  by  men 
taking  hold  of  the  capstan  bars  and  walking  around  with 
them,  or  by  attaching  an  endless  rope  or  chain  to  the  extremi- 
ties of  the  bars,  and  setting  it  in  motion  by  machinery. 

For  setting  disk-piles,  water  is  forced  down  through  the 
hole  in  the  disk,  and  produces  a  scour  from  under  the  pile 
which  gradually  sinks  to  its  place. 

The  manner  of  driving  piles,  and  the  extent  to  which  they 
may  be  forced  into  the  subsoil,  will  depend  on  local  circum- 
stances. It  sometimes  happens  that  a  heavy  blow  will  effect 
less  than  several  slighter  blows,  and  that  piles  after  an  inter- 
val between  successive  volleys  of  blows  can  with  difficulty  be 


200 


CIVIL  ENGES^EEESTG. 


started  at  first.  In  some  cases  the  pile  breaks  below  the  sur- 
face, and  continues  to  yield  to  the  blows  by  the  fibres  of  the 
lower  extremity  being  crushed.  These  difticulties  require 
careful  attention  on  the  part  of  the  engineer.  Piles  should 
be  driven  to  an  unyielding  subsoil.  The  French  civil  engi- 
neers have,  however,  adopted  a  rule  to  stop  the  driving  when 
the  pile  has  arrived  at  its  absolute  stojppage^  this  being  mea- 
sured by  the  further  penetration  into  the  subsoil  of  about 
y^ths  of  an  inch,  caused  by  a  volley  of  thirty  blows  from  a 
ram  of  800  lbs.,  falling  from  a  height  of  5  feet  at  each  blow. 

433.  If  the  head  of  a  pile  has  to  be  driven  below  the  level 
to  which  the  ram  descends,  another  pile,  termed  a  is 
used  for  the  purpose.  A  cast-iron  socket  of  a  suitable  form 
embraces  the  head  of  the  pile  and  the  foot  of  the  punch,  and 
the  effect  of  the  blow  is  thus  transmitted  through  the  punch 
to  the  pile. 

434.  "VVlien  a  pile,  from  breaking  or  any  other  cause,  has 
to  be  drawn  out,  it  is  done  by  using  a  long  beam  as  a  lever 
for  the  purpose ;  the  pile  being  attached  to  the  lever  by  a 
chain  or  rope,  suitably  adjusted. 

435.  The  number  of  piles  required  will  be  regulated  by 
the  weight  of  the  structure.  Where  the  piles  are  driven  to  a 
firm  subsoil,  they  may  be  subjected  to  a  working  strain  of 
1000  pounds  to  the  square  inch  of  cross  section  at  top.  In 
the  contrary  case,  and  where  the  resistance-  offered  results 
mainly  from  that  of  friction  on  the  exterior  of  the  piles,  the 
working  strain  should  be  reduced  to  200  pounds  to  the  square 
inch.  The  least  distance  apart  at  which  the  piles  can  be 
driven  with  ease  is  about  2J  feet  between  their  centres.  If 
they  are  more  crowded  than  this,  they  may  force  each  other 
up  as  they  are  successively  driven.  When  this  is  found  to 
take  place,  the  driving  should  be  commenced  at  the  centre  of 
the  area,  and  the  pile  should  be  driven  with  the  butt  end 
downward. 

436.  From  experiments  carefully  made  in  France,  it  appears 
that  piles  which  resist  only  in  virtue  of  the  friction  arising 
from  the  compression  of  the  soil,  cannot  be  subjected  with 
safety  to  a  load  greater  than  one-fifth  of  that  which  piles  of 
the  same  dimensions  will  safely  support  when  driven  into  a 
firm  soil. 

437.  After  the  piles  are  driven,  they  are  sawed  off  to  a 
level,  to  receive  a  grillage  and  platform  for  the  foundation. 
A  large  beam,  termed  a  capping^  is  first  placed  on  the  heads 
of  the  outside  row  of  piles,  to  which  it  is  fastened  by  means 
of  wooden  pins,  or  tree-iiails,  driven  into  an  auger-hole  made 


FOUNDATIONS  OF  STRUCTUKES. 


201 


through  the  cap,  into  the  head  of  each  pile.  After  the  cap  is 
fitted,  longitudinal  becims,  termed  string-jneces,  are  laid 
lengthwise  on  the  heads  of  each  row,  and  rest  at  each  extrem- 
ity on  the  cap,  to  which  they  are  fastened  by  a  dove-tail  joint 
and  a  wooden  pin.  Another  series  of  beams,  termed  cross- 
pieces^  are  laid  crosswise  on  the  string-pieces,  over  the  heads 
of  each  row  of  piles.  The  cross  and  string  pieces  are  con- 
nected by  a  notch  cut  into  each,  so  that,  when  put  together, 
their  upper  surfaces  may  be  on  the  same  level,  and  they  are 
fastened  to  the  heads  of  the  piles  in  the  same  manner  as  the 
capping.  The  extremities  of  the  cross-pieces  rest  on  the  cap- 
ping, and  are  connected  with  it  like  the  string-pieces. 

The  platform  is  of  thick  planks  laid  over  the  grillage,  with 
the  extremity  of  each  plank  resting  on  the  capping,  to  which, 
and  to  the  string  and  cross  pieces,  the  planks  are  fastened  by 
nails. 

The  capping  is  usually  thicker  than  the  cross  and  string 
pieces  by  the  thickness  of  the  plank ;  when  this  is  the  case,  a 
rabate,  about  four  inches  wide,  must  be  made  on  the  inner 
edge  of  the  capping,  to  receive  the  ends  of  the  planks. 

438.  An  objection  is  made  to  the  platform  as  a  bed  for  the 
foundation,  owing  to  the  want  of  adhesion  between  wood  and 
mortar ;  from  which,  if  any  unequal  settling  should  take 
place,  the  foundations  would  be  exposed  to  slide  off  the  plat- 
form. To  obviate  this,  it  has  been  proposed  to  replace  the 
grillage  and  platform  by  a  layer  of  beton  resting  partly  on 
the  heads  of  the  piles,  and  partly  on  the  soil  between  them. 
This  means  would  furnish  a  firm  bed  for  the  masonry  of  the 
foundations,  devoid  of  the  objections  made  to  the  one  of  tim- 
ber. 

To  counteract  any  tendency  to  sliding,  the  platform  may  be 
inclined  if  there  is  a  lateral  pressure,  as  in  the  case,  for  ex- 
ample, of  the  abutments  of  an  arch. 

439.  In  soils  of  alluvial  formation,  it  is  common  to  meet 
with  a  stratum  of  clay  on  the  surface,  underlaid  with  soft 
mud,  in  which  case  the  driving  of  short  piles  would  be  inju- 
rious, as  the  tenacity  of  the  stratum  of  clay  would  be  de- 
stroyed by  the  operation.  It  would  be  better  not  to  disturb 
the  upper  stratum  in  this  case,  but  to  give  it  as  much  firmness 
as  possible,  by  ramming  it  with  a  heavy  beetle,  or  by  submit- 
ting it  to  a  heavy  pressure. 

The  piers  of  the  bridge  over  the  Seekonk  river  are  formed 
of  clusters  of  piles  driven  through  the  mud  to  a  firm  subsoil. 

These  piles  are  of  hard  Southern  or  3'ellow  pine,  hewn  to 
twelve  or  fourteen  inches  square,  according  to  the  size  of  the 


202 


CIYIL  ENGINEERING. 


stick,  throughout  their  whole  length.    They  are  arranged  in 

groups  of  twelve,  except  in  five  clusters  under  the  draw, 
ight  of  the  piles  in  the  clusters  of  twelve  have  their  outside 
corners  taken  off  to  allow  the  flanges  of  the  cylinders  to  pass 


Fig.  39  represents  a  section  and  elevation  of  a  pier  of  the  Seekonk  river  bridgo. 

A,  outside  cover  of  metal. 

B,  clusters  of  wooden  piles. 

C,  inside  filling  of  coucrete. 

D,  loose  stone. 

E,  slag. 

F,  crust  of  shells. 

G,  cross  section  of  wooden  clusters. 

down  by  them.  The  piles  forming  each  of  these  clusters  are 
firmly  bolted  together  with  inch  and  a  quarter  bolts.  These 
clusters  are  incased  with  cast-iron  cylinders,  extending  from 
ten  inches  above  the  piling  in  the  draw  pier,  and  sixteen  or 


FOUNDATIONS  OF  STEUCTUEES. 


203 


twenty  inches  in  the  others,  to  four  and  six  feet  below  the  top 
of  the  crust.  The  cylinders  are  six  and  five  feet  in  diameter 
for  the  large  and  small  clusters,  and  the  void  space  left  be- 
tween them  and  the  clusters  is  filled  in  with  good  concrete. 

440.  Piles  and  sheeting  piles  of  cast  iron  have  been  used 
with  complete  success  in  England,  both  for  the  ordinary  pur- 
poses of  cofferdams,  and  for  permanent  structures  for  wharf- 
ing.  The  piles  have  been  cast  of  a  variety  of  forms ;  in  some 
cases  they  have  been  cast  hollow  for  the  purpose  of  excavat- 
ing the  soil  within  the  pile  as  it  was  driven,  and  thus  facili- 
tate its  penetration  into  the  subsoil.  Fig.  40  represents  a 
horizontal  section  of  one  of  the  more  recent  arrangements  of 
ii'on  piles  and  sheeting  piles. 


Fig.  40  represents  a  horizontal  section  of  an  arrangement  of  piles  and  sheeting  piles  of 
cast  iron. 

a,  sheeting  pile  with  a  lap  e  to  cover  the  joint  between  it  and  the  next  sheeting  pile. 
&,  piles  with  a  lap  on  each  side. 

c,  sheeting  pile  lapped  by  pile  and  sheeting  pile  next  it. 
tf,  ribs  of  piles  and  sheeting  piles. 

441.  Sand  has  also  been  used  with  advantage  to  form  a  bed 
for  foundations  in  a  very  compressible  soil.  For  this  purpose 
a  trench  is  (Fig.  40)  excavated,  and  filled  with  sand ;  the  sand 
being  spread  in  layers  of  about  9  inches,  and  each  layer  being 
firmly  settled  by  a  heavy  beetle,  before  laying  the  next.  If 


Fig.  41  represents  a  section  of  a  sand  fotm- 

dation  bed  and  the  masonry  upon  it. 

A,  sand  bed  in  a  trench. 

B,  masonry. 


water  should  make  rapidly  in  the  trench,  it  would  not  be 
practicable  to  pack  the  sand  in  layers.    Instead,  therefore,  of 


204 


CIVIL  ENGINEERING. 


opening  a  trench,  holes  about  6  feet  deep,  and  6  inches  in 
diameter  (Fig.  42),  should  be  made  by  means  of  a  short  pile, 
as  close  together  as  practicable ;  when  the  pile  is  withdrawn 
from  the  hole  it  is  immediately  filled  with  sand.  To  cause 
the  sand  to  pack  firmly,  it  should  be  slightly  moistened  before 
placing  it  in  the  holes  or  trench. 


Fig.  42. — Represents  a  section  of  a  foun- 
dation bed  made  by  filling  holes  with 
sand. 

A,  holes  filled  with  sand. 

B,  masonry. 


Sand,  when  used  in  this  way,  possesses  the  valuable  prop- 
erty of  assuming  a  new  position  of  equilibrium  and  stability, 
should  the  soil  on  which  it  is  laid  yield  at  any  of  its  points. 
'Not  only  does  this  take  place  along  the  base  of  the  sand  bed, 
but  also  along  the  edges,  or  sides,  when  these  are  enclosed  by 
the  sides  of  the  trench  made  to  receive  the  bed.  This  last 
point  offers  also  some  additional  security  against  yielding  in  a 
lateral  direction.  The  bed  of  sand  must,  in  all  cases,  receive 
sufiicient  thickness  to  cause  the  pressure  on  its  upper  sm-face 
to  be  distributed  over  the  entire  base. 

442.  AVlien,  from  the  fluidity  of  the  soil,  the  vertical  pres- 
sure of  the  structure  causes  the  soil  to  rise  around  the  bed, 
this  action  may  be  counteracted  either  by  scooping  out  the 
Boil  to  some  depth  around  the  bed  and  replacing  it  by  another 
of  a  more  compact  nature,  well  rammed  in  layers,  or  with  any 
rubbi&li  of  a  solid  character ;  or  else  a  mass  of  loose  stone 
may  be  placed  over  the  surface  exterior  to  the  bed,  whenever 
the  character  of  the  structure  will  warrant  the  expense. 

443.  Precautions  against  Lateral  Yielding.  The  soils 
w^hich  have  been  termed  compressible,  strictly  speaking,  yield 
only  by  the  displacement  of  their  particles  either  in  a  lateral 
direction,  or  upward  around  the  sti-ucture  laid  upon  them; 
"Where  this  action  arises  from  the  effect  of  a  vertical  weight, 
uniformly  distributed  over  the  base  of  the  bed,  the  preceding 


FOUNDATIONS  OF  STETJCTFEES. 


205 


methods  for  giving  permanent  stability  to  structure  present 
all  requisite  security.  But  when  the  structure  is  subjected 
also  to  a  lateral  pressure,  as,  for  example,  that  which  would 
arise  from  the  action  of  a  bank  of  earth  resting  against  the 
back  of  a  wall,  additional  means  of  security  are  demanded. 

One  of  the  most  obvious  expedients  in  this  case  is  to  drive 
a  row  of  strong  square  piles  in  juxtaposition  immediately  in 
contact  with  the  exterior  edges  of  the  bed.  This  expedient 
is,  however,  only  of  service  where  the  piles  attain  either  an 
incompressible  soil,  or  one  at  least  firmer  than  that  on  which 
the  bed  inmiediately  rests.  For  otherwise,  as  is  obvious,  the 
piles  only  serve  to  transmit  the  pressure  to  the  yielding  soil  in 
contact  with  them.  Bat  where  they  are  driven  into  a  firm 
soil  below,  they  gain  a  fixed  point  of  resistance,  and  the  only 
insecurity  they  otter  is  either  by  the  rupture  of  the  piles,  from 
the  cross  strain  upon  them,  or  from  the  yielding  of  the  firm 
subsoil,  from  the  same  cause. 

In  case  the  piles  reach  a  firm  subsoil,  it  will  be  best  to  scoop 
out  the  upper  yielding  soil  before  driving  the  piles  and  to  fill 
in  between  and  around  them  with  loose  broken  stone  (Fig.  43). 
This  will  give  the  piles  greater  stiffness,  and  effectually  pre- 
vent them  from  spreading  at  top. 


When  the  piles  cannot  be  secured  by  attaining  a  firm  sub- 
soil, it  will  be  better  to  drive  them  around  the  area  at  some 
distance  from  the  bed,  and,  as  a  further  precaution,  to  place 
horizontal  buttresses  of  masonry  at  regular  intervals  from  the 
bed  to  the  piles.  By  this  arrangement  some  additional  secu- 
rity is  gained  from  the  counter-pressure  of  the  soil  enclosed 


them  from  yielding  laterally. 

A,  section  of  the  masonry. 

B,  loose  stone  thrown  around  the  piles,  a. 


Fig.  43 — Represents  the  manner  of  using 


loose  stone  to  sustain  piles  and  prevent 


206 


CIVIL  ENGINEERING. 


between  the  bed  and  the  wall  of  piles.  But  it  is  obvious  that 
unless  the  piles  in  this  case  are  driven  into  a  firmer  soil  than 
that  on  which  the  structure  rests,  there  will  still  be  danger  of 
yielding. 

In  using  horizontal  buttresses,  the  stone  of  which  they  are 
constructed  should  be  dressed  with  care ;  their  extremities 
near  the  wall  of  piles  should  be  connected  by  horizontal 
arches  (Fig.  44),  to  distribute  the  pressure  more  uniformly ; 
and  where  there  is  an  upward  pressure  of  the  soil  around  the 

to  be 


structure,  arising  from  its  weight,  the  buttresses 


ought 


in  the  form  of  reversed  arches. 

In  buttresses  of  this  kind,  as  likewise  in  broad  areas  resting 
on  a  very  yielding  soil,  since  as  much  danger  is  to  be  appre- 
hended from  their  breaking  by  their  own  weight  as  from  any 
other  cause,  it  must  be  carefully  guarded  against.  Something 
may  be  done  for  this  purpose  by  ramming  the  earth  around 
the  structure  with  a  heavy  beetle,  when  it  can  be  made  more 
compact  by  this  means  ;  or  else  a  part  of  the  upper  soil  may 
be  removed,  and  be  replaced  by  one  of  a  more  compact  nature 
which  may  be  rammed  in  layers. 


if 

m 

m- 

-i 

Fig.  44  represents  the  manner  of  prevent- 
ing a  sustaining  wall  from  yielding  later- 
ally to  a  thrust  behind  it,  by  using  hori- 
zontal buttresses  of  reversed  arches  abut- 
ting against  vertical  counter-arches. 

A,  vertical  section  of  wall,  buttresses,  and 
counter-  arches. 

B,  plan  of  wall,  buttresses,  and  counter- 
arches. 

a,  plan  of  wall. 
6,  section  of  do. 

c,  buttresses. 

d,  counter- arches. 


The  following  methods,  where  they  can  be  resorted  to,  and 
where  the  character  of  the  structure  will  justify  the  expense, 
have  been  found  to  offer  the  best  security  in  the  case  in  ques- 
tion. 

AVhen  the  bed  can  be  buttressed  in  front  with  an  embank- 


FOUNDATIONS  OF  STETJCTTEES. 


207 


ment,  a  low  counter-wall  (Fig.  45)  may  be  built  parallel  to 
the  edge  of  the  bed, and  some  10  or  12  feet  from  it;  between 
this  wall  and  the  bed  a  reversed  arch  connecting  the  two  may 
be  built,  and  a  surcharge  of  earth  of  a  compact  character  and 
well  rammed,  may  be  placed  against  the  counter-wall  to  act 
by  its  counter-pressure  against  the  lateral  pressure  upon  the 

Fig.  45  represents  the  man- 
ner of  buttressing  a  sustain- 
ing wall  in  front  by  the  ac- 
tion of  a  counter-pressure  of 
earth  transmitted  to  the  wall 
by  a  reversed  arch, 
a,  section  of  sustaining  wall. 
&,  section  of  sustaining  wall 

of  embankment,  d. 
c,  section  of  reversed  arch, 
section   of  embankment 
from   which  counter-pres- 
sure comes, 
c,  section  of  embankment  be- 
hind sustaining  wall. 


"When  the  bed  cannot  be  buttressed  in  front,  as  in  quay 
walls,  a  grillage  and  platform  supported  on  piles  (Fig.  46) 
may  be  built  to  the  rear  from  the  back  of  the  wall,  for  the 
purpose  of  supporting  the  embankment  against  the  back  of 
the  wall,  and  preventing  the  effect  which  its  pressure  on  the 
subsoil  might  have  in  thrusting  forward  the  bed  of  the  founda- 
tion. 

In  addition  to  these  means,  land  ties  of  iron  will  give  great 
additional  security,  when  a  fixed  point  in  rear  of  the  wall  can 
be  found  to  attach  them  firmly. 


Fig,  46  represents  the  manner  of  re- 
lieving a  sustaining  wall  from  the 
lateral  action  caused  by  the  pressure 
of  an  embankment  on  the  subsoil  by 
means  of  a  platform  buUt  behind  the 
wall. 

A,  section  of  the  wall. 

B,  section  of  embankment. 

a,  piles  supporting  the  grillage  and  plat- 
form of  A. 

6,  loose  stone,  forming  a  firm  bed  under 
the  platforms. 

c,  piles  supporting  the  platform  d  behind 
the  waU. 


208 


CTVTL  ENGINEERING. 


YI. 

FOUNDATIONS  OF  STKUCTUEES  IN  WATER. 

444.  In  laying  foundations  in  water,  two  difficulties  liave 
to  be  overcome,  both  of  which  require  great  resources  and 
care  on  the  part  of  the  engineer.  The  first  is  found  in  the 
means  to  be  used  in  preparing  the  bed  of  the  foundation ; 
and  the  second  in  securing  the  bed  from  the  action  of  water, 
to  insure  the  safety  of  the  foundations.  The  last  is  generally 
the  more  difficult  problem  of  the  two ;  for  a  current  of  water 
will  gradually  wear  away,  not  only  every  variety  of  loose  soils, 
but  also  the  more  tender  rocks,  such  as  most  varieties  of  sand- 
stone, and  the  calcareous  and  argillaceous  rocks,  particularly 
when  they  are  stratified,  or  are  of  a  loose  texture. 

445.  To  prepare  the  bed  of  a  foundation  in  stagnant  water 
the.  only  difficulty  that  presents  itself  is  to  exclude  the  water 
from  the  area  on  which  the  structure  is  to  rest.  If  the  depth 
of  water  is  not  over  4  feet,  this  is  done  by  surrounding  the 
area  with  an  ordinary  water-tight  dam  of  clay,  or  of  some 
other  binding  earth.  For  this  purpose,  a  shallow  trench  is 
formed  around  the  area,  by  removing  the  soft  or  loose  stratum 
on  the  bottom  ;  the  foundation  of  the  dam  is  commenced  by 
filling  this  trench  with  the  clay,  and  the  dam  is  made  by 
spreading  successive  layers  of  clay  about  one  foot  thick,  and 
pressing  each  layer  as  it  is  spread  to  render  it  more  compact, 
when  the  dam  is  completed,  the  water  is  pumped  out  from 
the  enclosed  area,  and  the  bed  for  the  foundation  is  prepared 
as  on  dry  land. 

446.  When  the  depth  of  stagnant  water  is  over  4  feet,  and 
in  running  water  of  any  depth,  the  ordinary  dam  must  be 
replaced  by  the  coffer-dam.  This  construction  consists  of 
two  rows  of  plank,  termed  sheeting  jpiles^  driven  into  the  soil 
vertically,  forming  thus  a  coffer-work,  between  which  clay  or 
binding  earth,  termed  the  puddling,  is  filled  in,  to  form  a 
water-tight  dam  to  exclude  the  water  from  the  area  enclosed. 

The  arrangement,  construction,  and  dimensions  of  coffer- 
dams depend  on  their  specific  object,  the  depth  of  water,  and 
the  nature  of  the  subsoil  on  which  the  coffer-dam  rests. 

With  regard  to  the  first  point,  the  width  of  the  dam  be- 
tween the  sheeting  piles  should  be  so  regulated  as  to  serve  as 
a  scaffoldino^  for  the  machinery  and  materials  required  about 
the  work.  I'his  is  peculiarly  requisite  where  the  coffer-dam  en- 


FOUNDATIONS  OF  STRTJCTUEES. 


209 


closes  an  isolated  position  removed  from  the  shore.  The 
interior  space  enclosed  by  the  dam  should  have  the  requisite 
capacity  for  receiving  the  bed  of  the  foundations,  and  such 
materials  and  machinery  as  may  be  required  within  the  dam. 

The  width  or  thickness  of  the  cotfer-dam,  by  which  is 
understood  the  distance  between  the  sheeting  piles,  should  be 
sufficient  not  only  to  be  impermeable  to  water,  but  to  form, 
by  the  weight  of  the  puddling,  in  combination  with  the  resis- 
tance of  the  timber- work,  a  wall  of  sufficient  strength  to  resist 
the  horizontal  pressure  of  the  water  on  the  exterior,  when  the 
interior  space  is  pumped  dry.  The  resistance  offered  by  the 
weight  of  the  puddling  to  the  pressure  of  the  water  can  be 
easily  calculated ;  that  offered  by  the  timber-work  will  depend 
upon  the  manner  in  which  the  framing  is  arranged,  and  the 
means  taken  to  stay  or  buttress  the  dam  from  the  enclosed 
space. 

The  most  simple  and  the  usual  construction  of  a  coffer-dam 


(Fig.  47)  consists  in  driving  a  row  of  ordinary  straight  piles 
around  the  area  to  be  enclosed,  placing  their  centre  lines  about 
4  feet  asunder.    A  second  row  is  driven  parallel  to  the  first, 
the  respective  piles  being  the  same  distance  apart ;  the  dis- 
tance between  the  centre  lines  of  the  two  rows  being  so  regu- 
lated as  to  leave  the  requisite  thickness  between  the  sheeting 
iles  for  the  dam.    The  piles  of  each  row  are  connected  by  a 
orizontal  beam  of  square  timber,  termed  a  string  or  wale 
jpiece,  placed  a  foot  or  two  above  the  highest  water  line,  and 
14 


210 


CIVIL  ENGINEETilNG. 


notched  and  bolted  to  each  pile.  The  string  pieces  of  the 
inner  row  of  piles  are  placed  on  the  side  next  to  the  area 
enclosed,  and  those  of  the  outer  row  on  the  outside.  Cross 
beams  of  square  timber  connect  the  string  pieces  of  the  two 
rows  upon  which  they  are  notched,  serving  both  to  prevent 
the  rows  of  piles  from  spreading  from  the  pressure  that  may 
be  thrown  on  them  and  as  a  joisting  for  the  scaffolding.  On 
the  opposite  sides  of  the  rows  interior  string  pieces  are  placed, 
about  the  same  level  with  the  exterior,  for  the  purpose  of 
serving  both  as  guides  and  supports  for  the  sheeting  piles. 
The  sheeting  piles  being  well  jointed  are  driven  in  juxtaposi- 
tion, and  against  the  interior  string  pieces.  A  third  course 
of  string  or  rihhon  pieces  of  smaller  scantling  confine,  by 
means  of  large  spikes,  the  sheeting  piles  against  the  interior 
string  pieces. 

As  has  been  stated,  the  thickness  of  the  dam  and  the  dimen- 
sions of  the  timber  of  which  the  coifer-work  is  made  will  de- 
pend upon  the  pressure  due  to  the  head  of  water,  when  the 
interior  space  is  pumped  dry.  For  extraordinary  depths,  the 
engineer  would  not  act  prudently  were  he  to  neglect  to  verify 
by  calculation  the  equilibrium  between  the  pressure  and  re- 
sistance ;  but  for  ordinary  depths  under  10  feet,  a  rule  fol- 
lowed is  to  make  the  thickness  of  the  dam  10  feet ;  and  for 
depths  over  10  feet  to  give  an  additional  thickness  of  one  foot 
for  every  additional  depth  of  three  feet.  This  rule  will  give 
every  security  against  tiltrations  through  the  body  of  the  dam, 
but  it  might  not  give  sufficient  strength  unless  the  scantling 
of  the  coffer- work  were  suitably  increased  in  dimensions. 

In  very  deep  tidal  water,  coffer-dams  have  been  made  in 
offsets,  by  using  three  rows  of  sheeting  piles  for  the  purpose 
of  giving  greater  thickness  to  the  dam  below  the  low-water 
level.  In  such  cases  strong  square  piles  closely  jointed  and 
tongued  and  grooved,  should  be  used  in  place  of  the  ordinary 
sheeting  piles. 

Besides  providing  against  the  pressure  of  the  head  of  water, 
suitable  dimensions  must  be  given  to  the  sheeting  piles,  in 
order  that  they  may  sustain  the  pressure  arising  from  the  pud- 
dling when  the  interior  space  is  emptied  of  water.  This 
pressure  against  the  interior  sheeting  piles  may  be  further 
increased  by  that  of  the  exterior  water  upon  the  exterior 
sheeting  ])iles,  should  the  pressure  of  the  latter  be  greater 
than  the  former.  To  provide  more  securely  against  the  effect 
of  these  pressures,  intermediate  string  pieces  may  be  placed 
against  the  interior  row  of  piles  before  the  sheeting  piles  are 
driven ;  and  the  opposite  sides  of  the  dam  on  the  interior  may 


FOUNDATIONS  OF  STEUCTUEE8. 


211 


be  buttressed  by  cross  pieces  reaching  across  the  top  string 
pieces,  and  by  horizontal  beams  placed  at  intermediate  points 
between  the  top  and  bottom  of  the  dam. 

The  main  inconvenience  met  with  in  coffer-dams  arises 
from  the  difficulty  of  preventing  leakage  under  the  dam.  In 
all  cases  the  piles  must  be  driven  into  a  firm  stratum,  and  the 
sheeting  piles  should  equally  have  a  firm  footing  in  a  tena- 
cious compact  substratum.  When  an  excavation  is  requisite 
on  the  interior,  to  uncover  the  subsoil  on  which  the  bed  of  the 
foundation  is  to  be  laid,  the  sheeting  piles  should  be  driven 
at  least  as  deep  as  this  point,  and  somewhat  below  it  if  the 
resistance  offered  to  the  driving  does  not  prevent  it. 

The  puddling  should  be  formed  of  a  mixture  of  tenacious 
clay  and  sand,  as  this  mixture  settles  better  than  pure  clay 
alone.  Before  placing  the  puddling,  all  the  soft  mud  and 
loose  soil  between  the  sheeting  piles  should  be  carefully  ex- 
tracted ;  the  puddling  should  be  placed  in  and  compressed  in 
layers,  care  being  taken  to  agitate  the  water  as  little  as  prac- 
ticable. 

With  requisite  care  coffer-dams  may  be  used  for  founda- 
tions in  any  depth  of  water,  provided  a  w^ater-tight  bottoming 
can  be  found  for  the  puddling.  Sandy  bottoms  offer  the 
greatest  difficulty  in  this  respect,  and  when  the  depth  of 
water  is  over  5  feet,  extraordinary  precautions  are  requisite 
to  prevent  leakage  under  the  puddling. 

When  the  depth  of  water  is  over  10  feet,  particularly  where 
the  bottom  is  composed  of  several  feet  of  soft  mud,  or  of  loose 
soil,  below  which  it  will  be  necessary  to  excavate  to  obtain  a 
firm  stratum  for  the  bed  of  the  foundation,  additional  precau- 
tions will  be  requisite  to  give  sufficient  support  to  the  interior 
sheeting  piles  against  the  pressure  of  the  puddling,  to  provide 
against  leakage  under  the  puddling,  and  to  strengthen  the 
dam  against  the  pressure  of  the  exterior  w^ater,  when  the  inte- 
rior space  is  pumped  dry  and  excavated.  The  best  means  for 
these  ends,  when  the  locality  will  admit  of  their  application, 
is  to  form  the  exterior  of  the  dam,  as  has  already  been  de- 
scribed, by  using  piles  and  sheeting  piles,  giving  to  the  latter 
additional  points  of  support,  by  intermediate  string  pieces 
between  the  one  at  top  and  the  bottom  of  the  water  ;  and  to 
form  a  strong  framing  of  timber  for  a  support  to  the  interior 
sheeting  piles,  giving  to  it  the  dimensions  of  the  area  to  be 
enclosed.  The  framework  (Fig.  48)  may  be  composed  of 
upright  square  beams,  placed  at  suitable  distances  apart,  de- 
pending on  the  strength  required,  upon  which  square  string 
pieces  are  bolted  at  suitable  distances  from  the  top  to  the 


212 


CIVIL  ENGINEEKING. 


bottom,  the  bottom  string  resting  on  the  surface  of  the  mud. 
The  string  j^ieces,  serving  as  supports  for  the  sheeting  piles, 
must  be  on  the  sides  of  the  uprights  towards  the  puddling, 
and  their  faces  in  the  same  vertical  plane.    Between  each 


Fig.  48  Represents  a 
section  of  the  coffer- 
dam used  for  the 
Potomac  aqueduct. 

a,  main  exterior  piles. 

6,  strong  square  beams 
correr:ponding  to  a 
on  which  the  wales 
n,  n  are  notched  and 
bolted. 

c,  sheeting  piles. 

d,  top  wale  on  main 
piles. 

6,  crosspieces. 

i,  guide  and  supporting 

string    i^ieces  for 

sheeting  piles. 
00,  horizontal  shores 

buttressing  opposite 

sides  of  dam. 

A,  puddling. 

B,  interior  space. 

C,  mud,  etc. 

D,  rock  bottom. 


pair  of  opposite  uprights  horizontal  shores  may  be  placed  at 
the  points  opposite  the  position  of  the  string  pieces,  to  in- 
crease the  resistance  of  the  dam  to  the  pressure  of  the  water ; 
the  top  shores  extending  entirely  across  the  dam,  and  being 
notched  on  the  top  string  pieces.  The  interior  shores  must 
be  so  arranged  that  they  can  be  readily  taken  out  as  the  ma- 
sonry on  the  interior  is  built  up,  replacing  them  by  other 
shores  resting  against  the  masonry  itself. 

447.  Caisson  and  Cribwork  Coffer-dams.  In  the  con- 
struction of  the  foundations  for  the  piers  and  abutments  of 
the  Victoria  tubular  iron  railroad  bridge  over  the  river  Saint 
Lawrence,  at  Montreal,  the  engineers  had  to  contend  against 
unusual  difficulties  ;  in  a  rocky  bottom  covered  with  boulders, 
which  prevented  the  use  of  piles ;  and  in  a  swift  current, 
bringing  down  in  the  spring  of  the  year  enormous  iields  of 
ice,  the  effects  of  which  none  of  the  ordinary  methods  of 
caisson  or  coifer-dams  could  have  withstood. 

These  difficulties  were  successfully  met,  in  some  cases  by 
the  use  of  a  large  water-tight  caisson,  shown  in  plan  (Fig.  49), 
and  in  cross-section  (Fig.  51),  of  such  a  form  and  dimensions 
as  to  leave  a  sufficient  interior  area,  between  its  interior  sides, 


FOUNDATIONS  OF  STEUCTTJEES. 


213 


for  a  coffer-dam,  and  for  the  ground  to  be  occupied  for  the 
construction  of  the  foundations  of  the  pier.  In  others  (Fig. 
51),  where,  from  the  velocity  of  the  current,  the  caissons, 
from  their  great  bulk,  proved  unmanageable,  by  enclosing  the 
area  to  be  occupied  by  crib-work,  sunk  upon  the  bottom  and 
heavily  laden  with  stone ;  and  exterior  to  this  forming  a 
second  similar  enclosure ;  and  then,  by  means  of  sheeting 
piles,  supported  against  the  opposite  sides  of  these  two  en- 
closures, forming  a  coffer-dam  between  them. 


I 

i 


I 


Fig.  49.    Plan  of  caisson.         B,  Detached  end. 
A,  A,  sides  of  caisson.  C,  Puddling. 

D,  Plan  of  pier  of  bridge. 

The  caisson  (Fig.  49)  consisted  of  two  parts,  the  two  sides 
and  up-stream  wedge-shaped  head,  and  a  rectangular-shaped 
portion  B,  which  fitted  in  between  the  two  sides,  forming  the 
down-stream  end,  and  which  could  be  detached  and  floated 
off  when  it  became  necessary  to  remove  the  entire  caisson. 

The  caisson  (Fig.  50)  was  flat-bottomed,  with  vertical  sides ; 
and  it  was  provided  with  a  strong  flat  deck,  to  receive  the 
workshops,  machinery,  and  materials  for  pumping,  dredging, 
and  the  construction  of  the  masonry. 

When  placed  in  position,  it  was  moored  to  a  loaded,  sunken 
crib-work  up-stream ;  and,  besides  the  exterior  guide-piles, 
long  two-inch  iron  bolts  were  inserted  into  holes  drilled  into 
the  solid  rock,  through  vertical  holes  bored  through  the  piles. 
In  this  way,  through  the  bearing  of  the  piles  on  the  bottom, 
the  foothold  given  by  the  bolts  and  the  mooring-tackle,  the 


214 


CIVIL  ENGENEEEING. 


caisson,  when  sunk,  was  solidly  secured  against  accidents  from 
rafts,  or  other  floating  bodies. 


Fig.  50.  Cross-section  of  Fig,  49.  C,  Cross-section  of  puddling  and  sheeting. 
A',  Cross-section  of  caisson.  D',  Foundation  courses  of  pier. 


The  interior  sides  of  the  coffer-dam  were  strongly  buttress- 
ed by  horizontal  beams,  to  withstand  the  pressure  of  the  water. 
These  beams  were  removed,  and  their  places  supplied  by 
shorter  buttresses  placed  between  the  sides  of  the  coffer-dam 
and  pier  as  the  masonr}^  was  carried  up. 

The  cribwork  dams  were  constructed  of  a  number  of  cribs, 
each  about  forty  feet  in  length,  which  w^ere  placed  end  to  end 
to  form  the  sides  of  the  enclosures,  and  strongly  connected 
with  each  other.  Some  of  these  were  constructed  on  shore, 
and  towed  to  their  positions.  Some  were  constructed  in  the 
water  behind  mooring  cribs,  and  others  upon  the  ice  during 
the  winter,  and  sunk  in  j^osition. 

A  flooring  (Fig.  51)  was  made,  about  midway  between  the 
top  and  bottom  of  the  cribs,  to  receive  tlie  blocks  of  stone  with 
which  the  cribs  were  loaded,  to  secure  them  from  the  effects 
of  the  pressure  of  tlie  ice  in  its  spring  movement,  and  the 
collision  of  floating  bodies. 

The  caissons  were  not  of  adequate  strength  to  resist  the 
crush  of  the  ice,  and  had  to  be  pmnped  out  and  removed  to  a 
secure  position  before  the  closing  of  the  river.  The  cribs 
w^ere  planked  over  at  top,  and  remained  in  place  as  long  as  re- 
quired for  the  work. 

448.  When  the  bed  of  a  river  presents  a  rocky  surface,  or 


FOUNDATIONS  OF  STRIJCTUEES. 


215 


rock  covered  with  but  a  few  feet  of  mud,  or  loose  soil,  cases 
may  occur  in  which  it  will  be  more  economical  and  equally 
safe  to  lay  a  bed  of  beton  without  exhausting  the  water  from 
the  area  to  be  built  on ;  enclosing  the  area,  before  throwing 
in  the  beton,  by  a  simple  coffer-work  formed  of  a  strong 


B 


Fig.  51.  Cross- section  of  cribwork  dams.         B,  Exterior  crib. 

A,  Interior  crib.  C,  Puddling  and  sheeting  piles. 


framework  of  uprights  and  horizontal  beams  and  sheeting 
piles.  The  framework  (Fig.  52)  in  this  case  is  composed  of 
uprights  connected  by  stri]ig  pieces  in  pairs;  each  pair  is 
notched  and  bolted  to  the  uprights,  a  sufficient  interval  being 
left  between  them  for  the  insertion  of  the  sheeting  piles.  To 
secure  the  framework  to  the  rock,  it  may  be  requisite  to  drill 
holes  in  the  rock  to  receive  the  foot  of  each  upright.  The 
holes  may  be  drilled  by  means  of  a  long  iron  bar,  termed  a 
jumper,  which  is  used  for  this  purpose,  or  else  the  ordinary 
diving-bell  may  be  employed.  This  machine  is  very  service- 
able in  all  similar  constructions  where  an  examination  of 
work  under  water  is  requisite,  or  in  cases  where  it  is  neces- 
sary to  lay  masonry  under  water.  The  framework  is  put 
together  on  land,  floated  to  its  position,  and  settled  upon  the 
rock ;  the  sheeting  piles  are  then  driven  into  close  contact 
with  the  surface  of  the  rock. 

449.  The  convenience  and  economy  resulting  from  the  use 
of  beton  for  the  beds  of  structures  raised  in  water  have  led 
General  Treussart  to  propose  a  plan  for  laying  beds  of  this 
material,  and  then  to  take  advantage  of  their  strength  and 
impermeability  to  construct  a  coffer-dam  upon  them,  m  order 
to  carry  on  the  superstructure  wdth  more  care.  To  effect 
this,  the  space  to  be  occupied  by  the  bed  (Fig.  53)  is  first  en- 
closed by  square  piles,  driven  in  juxtaposition  and  secured  at 
top  by  a  string  piece.  The  mud  and  loose  soil  are  then 
scooped  from  the  enclosed  area  to  the  firm  soil  on  which  the 


216 


CIVIL  ENGINEEEENG. 


bed  of  beton  is  to  be  laid.  The  bed  of  beton  is  next  laid 
with  the  usual  precautions,  and  while  it  is  still  soft  a  second 
row  of  square  piles  is  driven  into  it,  also  in  juxtaposition,  and 


Fig.  52  represents  a  coffer- work  for  confining  b^ton. 

A,  Section  of  coffer-work  and  bdton. 

B,  Plan  of  coffer-work. 

a,  a',  square  uprights  connected  by  horizontal  beams,  6  &, 
bolted  to  them  in  pairs. 

c,  c',  sheeting  piles  inserted  between  the  beams  6,  b'  and 
the  uprights  a,  a'. 

d,  d\  iron  rods  connecting  opposite  sides  of  coffer-work. 


at  a  suitable  distance  from  the  first  for  the  thickness  of  the 
dam ;  these  are  also  secured  at  top  by  a  string  piece.  Cross 
pieces  are  notched  upon  the  string  pieces,  to  secure  the  rows  of 
piles  and  form  a  scaffolding.  An  ordinary  puddling  is  placed 
in  between  the  rows  of  piles,  and  the  interior  space  is  pumped 
dry. 

Should  the  soil  under  the  bed  of  beton  be  permeable,  the 
pressure  of  the  water  on  the  base  of  the  bed  may  be  sufficient 
to  raise  the  bed  and  the  dam  upon  it,  when  the  water  is  taken 
from  the  interior  space.  A  proper  calculation  will  show 
whether  this  danger  is  to  be  apprehended,  and  should  it  be, 


FOUNDATIONS  OF  STEUCTTJEES. 


217 


a  provisional  weight  mirst  be  placed  on  the  dam,  or  the  bed 
of  beton,  before  exhausting  the  interior. 


Fig.  53  represents  a  section  of  Gen- 
eral Treussart's  dam. 

A,  bed  of  beton. 

B,  puddling  of  dam. 

C,  masonry  of  structure. 

a,  square  piles. 

b,  wale  pieces. 

c,  cross  pieces. 


450.  When  the  depth  of  water  is  great,  or  when,  from  the 
permeability  of  the  soil  at  the  bottom,  it  is  difficult  to  pre- 
vent leakage,  a  coffer-dam  may  be  a  less  economical  method 
of  laying  foundations  than  the  caisson.  The  caisson  (fig.  54.) 
is  a  strong  water-tight  vessel  having  a  bottom  of  solid  heavy 
timber,  and  vertical  sides  so  arranged  that  they  can  be  readi- 
ly detached  from  the  bottom.  'The  following  is  the  usual 
arrangement  of  the  caisson,  it,  like  the  coffer-dam,  being  sub- 
ject to  changes  to  suit  it  to  the  locality.  The  bottom  of  the 
caisson,  serving  as  a  platform  for  the  foundation  course  of 
the  masonry,  is  made  level  and  of  heavy  timber  laid  in  juxta- 
position, the  ends  of  the. beams  being  confined  by  tenons  and 
Bcrew-bolts  to  longitudinal  capping  pieces- of  larger  dimen- 
sions. The  sides  of  the  box  are  usually  vertical.  The  sides 
are  formed  of  upright  pieces  of  scantling  covered  with  thick 
piank,  the  seams  being  carefully  calked  to  make  the  caisson 
water-tight.  The  lower  ends  of  the  uprights  are  inserted 
into  shallow  mortises  made  in  the  capping.  The  arrange- 
ment for  detaching  the  sides  is  effected  in  the  following 
manner :  Strong  hooks  of  wrought  iron  are  fixed  to  the  bot- 
tom of  the  caisson  at  the  sides  of  the  capping  piece,  corre- 
sponding to  the  points  where  the  uprights  of  the  sides  are  in- 


218 


CIYIL  ENGINEERING. 


serted  into  this  piece.  Pieces  of  strong  scantling  are  laid 
across  the  top  of  the  caisson,  resting  on  the  opposite  uprights, 
upon  which  they  are  notched.  These  cross  pieces  project 
bej^ond  the  sides,  and  the  projecting  parts  are  perforated  by 
an  auger-hole,  large  enough  to  receive  a  bolt  of  t^vo  inches  in 


Tig.  54  represents  a  cross 
section  and  interior  end 
view  of  a  caisson.  The 
bf)ar(Is  in  this  figure  are  rep- 
resented as  let  into  grooves 
in  the  vertical  pieces,  in- 
stead of  being  nailed  to  them 
on  the  exterior. 

CP,  bottom  bean»s  let  into 
groo-s-es  in  the  capping. 

6,  square  uprights  to  sustain 
the  boards. 

c,  cross  pieces  resting  on  6. 

d,  iron  rotls  fitted  to  hooks  all 
bottom  and  nnts  at  top. 

e,  longitudinal  beams  to  stay 
the  cross  pieces  c. 

A,  section  of  the  masonry. 


diameter.  The  object  of  these  cross  pieces  is  twofold ;  the 
first  is  to  buttress  the  sides  of  the  caisson  at  top  against  the 
exterior  pressure  of  the  water  ;  and  the  second  is  to  serve  as 
a  point  of  support  for  a  long  bolt^  or  rod  of  iron,  with  an  eye 
at  the  lower  end,  into  which  the  hook  on  the  capping  piece  is 
inserted,  and  a  screw  at  top,  to  which  a  nut  or  female  screw 
is  fitted,  and  which,  resting  on  the  cross  pieces  as  a  point  of 
support,  draws  the  bolt  tight,  and,  in  that  way,  attaches  the 
sides  and  bottom  of  the  caisson  firmly  together. 

A  bed  is  prepared  to  receive  the  bottom  of  the  caisson,  by 
levelling  the  soil  on  which  the  structure  is  to  rest,  if  it  be  of 
a  suitable  character  to  receive  directly  the  foundation ;  or  by 
driving  large  piles  through  the  upper  compressible  strata  of 
the  soil  to  the  firm  stratum  beneath.  Tlie  heads  of  the  piles 
are  sawed  off  on  a  level  to  receive  the  bottom  of  the  caisson. 

To  settle  the  caisson  on  its  bed,  it  is  floated  to  and  moored 
over  it ;  and  the  masomy  of  the  structure  is  commenced  and 
carried  up,  until  the  weight  grounds  the  caisson.  Tlie  caisson 
should  be  so  contrived,  tliat  it  can  be  grounded,  and  after- 
wards raised,  in  case  that  the  bed  is  found  not  to  be  accurately 
levelled.  To  effect  this,  a  small  sliding  gate  should  be  placed 
in  the  side  of  the  caisson,  for  the  purpose  of  filling  it  with 


FOTHSTDATIONS  OF  STEUCTUEES. 


219 


water  at  pleasure.  By  means  of  this  gate,  the  caisson  can  be 
filled  and  grounded,  and  by  closing  the  gate  and  pumping  out 
the  water,  it  can  be  set  afloat. 

After  the  caisson  is  settled  on  its  bed,  and  the  masonry  of 
the  structure  is  raised  above  the  surface  of  the  water,  the  sides 
are  detached  by  first  unscrewing  the  nuts  and  detaching  the 
rods  and  then  taking  oif  the  top  cross  pieces.  By  first  filling 
the  caisson  with  water,  this  operation  of  detaching  the  sides 
can  be  more  easily  performed. 

451.  To  adjust  the  piles  before  they  are  driven,  and  to  pre- 
vent them  from  spreading  outward  by  the  operation  of  driving, 
a  strong  grating  of  heavy  timber,  formed  by  notching  cross 
and  longitudinal  pieces  on  each  other,  and  fastening  them 
firmly  together,  may  be  resorted  to.  This  grating  is  arranged 
in  a  similar  maimer  to  a  grillage ;  only  the  square  compart- 
ments between  the  cross  and  string  pieces  are  larger,  so  that 
they  may  enclose  an  area  for  4  or  9  piles ;  and  instead  of  a 
single  row  of  cross  pieces,  the  grating  is  made  with  a  double 
row,  one  at  top,  the  other  at  the  bottom,  embracing  the  string 
pieces  on  which  they  are  notched. 

The  grating  may  be  fixed  in  its  position  at  any  depth  under 
water,  by  a  few  provisional  piles,  to  which  it  can  be  attached. 

452.  Where  the  area  occupied  by  a  structure  is  very  con- 
siderable, and  the  depth  of  water  great,  the  methods  which 
have  thus  far  been  explained  cannot  be  used.  In  such  cases, 
a  firm  bed  is  made  for  the  structure,  by  forming  an  artificial 
island  of  loose  heavy  blocks  of  stone,  which  are  spread  over 
the  area,  and  receive  a  batter  of  from  one  perpendicular  to 
one  base,  to  one  perpendicular  and  six  base,  according  to  the 
exposure  of  the  bed  to  the  effects  of  waves.  This  bed  is 
raised  several  feet  above  the  surface  of  the  water,  according 
to  the  nature  of  the  structure,  and  the  foundation  is  com- 
menced upon  it. 

453.  It  is  important  to  observe,  that,  where  such  heavy  masses 
are  laid  upon  an  untried  soil,  the  structure  should  not  be  com- 
menced before  the  bed  appears  entirely  to  have  settled  ;  nor 
even  then  if  there  be  any  danger  of  further  settling  taking 
place  from  the  additional  weight  of  the  structure.  Should 
any  doubts  arise  on  this  point,  the  bed  should  be  loaded  with 
a  provisional  w^eight,  somewhat  greater  than  that  of  the  con- 
templated structure,  and  this  weight  may  be  gradually  re- 
moved, if  composed  of  other  materials  than  those  required 
for  the  structure,  as  the  work  progresses. 

454.  To  give  perfect  security  to  foundations  in  running 
water,  the  soil  around  the  bed  must  be  protected  to  some  ex- 


220 


CIVIL  ENGINEERING. 


tent  from  the  action  of  the  current.  The  most  ordinary 
method  of  effecting  this  is  by  throwing  in  loose  masses  of 
bi'oken  stone  of  sufficient  size  to  resist  the  force  of  the  cur- 
rent. This  method  will  give  all  required  security,  where  the 
soil  is  not  of  a  shifting  character,  like  sand  and  gravel.  To 
secure  a  soil  of  this  last  nature,  it  will  in  some  cases  be  neces- 
sary to  scoop  out  the  bottom  around  the  bed  to  a  depth  of 
from  3  to  6  feet,  and  to  till  this  excavated  part  with  beton, 
the  surface  of  which  may  be  protected  from  the  wear  arising 
from  the  action  of  the  pebbles  carried  over  it  by  the  current, 
by  covering  it  with  broad  fiat  flagging  stones. 

455.  When  the  bottom  is  composed  of  soft  mud  to  any 
great  depth,  it  may  be  protected  by  enclosing  the  area  with 
sheeting  piles,  and  then  filling  in  the  enclosed  space  with  frag- 
ments of  loose  stone.  If  the  mud  is  very  soft,  it  would  be 
advisable,  in  the  first  place,  to  cover  the  area  with  a  grillage, 
or  with  a  layer  of  brushwood  laid  compactly,  to  serve  as  a 
bed  for  the  loose  stone,  and  thus  form  a  more  stable  and  solid 
mass. 

456.  Pneumatic  Processes. — By  this  term  we  understand 
those  methods  of  obtaining  foundations  in  water,  in  which 
external  or  internal  atmospheric  pressure  is  the  active  agent. 

Tliese  processes  are  divided  into  two  classes,  viz. :  the 
j)lenum  2>neumatic  and  the  vacuum  pneicmatic,  the  former 
term  being  applied  to  the  case  where  the  pressure  of  con- 
densed air  is  employed  to  drive  the  water  out,  and  the  latter, 
w^here  the  pressure  of  the  atmosphere  is  employed  to  drive 
the  water  into  a  vacuum. 

457.  Pneumatic  Piles. — This  appellation  has  been  given 
to  cylinders  of  cast-iron,  used  in  the  place  of  ordinary  piles  to 
reach  a  firm  subsoil  below  the  bed  of  a  river,  suital)]e  for  the 
character  of  the  superstructure  to  rest  u])on  it,  which,  being 
made  air-tight  on  the  sides  and  top,  but  left  open  at  the  bot- 
tom, are  sunk  to  the  required  depth,  by  rapidly  withdrawing 
the  air  within  them,  by  methods  to  Ibe  described,  and  thus 
causing  the  water  to  rush  in  through  the  open  bottc^m,  remo- 
ving in  its  flow  the  subsoil  in  contact  with  the  lower  end  of 
the  cylinder,  and  allowing  it  to  sink  by  its  own  weight,  thus 
belonging  to  the  vacuum  pneumatic  class. 

The  cylinders  are  cast  and  put  together  very  much  in  the 
same  manner  as  ordinary  water-pipes ;  being  composed  of 
lengths  of  from  six  to  ten  feet,  each  of  which  has  an  interior 
flange  at  each  end,  with  holes  for  screw-bolts,  by  means  of 
whicli  and  a  disk  of  india-rul)ber,  inserted  between  the  con- 
necting flanges,  the  joints  are  made  air  and  water  tight. 


FOUNDATIONS  OF  STKUCTUEES. 


221 


In  tne  first  essays  at  this  mode  of  foundation,  the  cylinders 
were  sunk  by  simply  exhausting  the  internal  air,  in  the  ordi- 
nary way,  above  the  water-level.  The  results,  however,  were 
not  satisfactory,  as  the  pile  sunk  very  slowly. 

The  next  step  (Fig.  55)  was  to  connect  an  air-tight  cylin- 
drical vessel,  D,  by  means  of  a  tube  A,  with  a  stop-cock, 
with  the  interior  of  the  pile  A,  and  also  with  the  air-pump, 
by  another  tube  leading  to  the  pump  from  the  other  end.  In 
order  to  sink  the  lA\e,  the  communication  between  it  and  the 
exhaust  chamber  D  was  first  closed,  and  that  between  this 
chamber  and  air-pump  opened.  The  air  was  then  drawn 
from  D  until  a  sufiicient  vacuum  was  produced,  when  the 


M   


Fig.  55. — Longitudinal  sectiort 
of  a  pneumatic  pile  A,  air* 
lock  C,  and  exhaust  vessel 
D. 

A,  exhaust  tube  between  A 
and  D. 

B,  water  discharge-tube. 

0,  equilibrium  tube  between 

the  lock  and  chamber  of 

the  pile. 
D,  equilibrium  tube  between 

lock  and  exterior  air. 
M,   upper     man-hole  and: 

valves. 

N,  lower     man-hole  and 

valves. 
W,  windlass  and  gearing, 
B,  concrete  underpinning  as 

practised  at  Harlem  bridge.. 


communion  with  the  pump  was  closed,  and  that  with  the  pile 
opened,  allowing  the  air  to  flow  from  it  into  the  chamber  with 
considerable  velocity.  This  sudden  disturbance  of  the  equi- 
librium between  the  external  and  internal  pressures  on  the 


222 


CIVIL  ENGINEERING. 


pile  caused  it  to  descend  instantaneously  and  rapidly,  as  if 
Btruck  on  the  top  by  a  heavy  blow,  the  descent  continuing 
frequently  many  feet  until  an  equilibrium  among  the  forces 
w  as  restored. 

When  the  resistance  to  the  further  descent  of  the  pile  was 
found  to  be  too  great,  either  from  some  obstruction  met  with 
at  the  bottom,  or  from  the  tenacity  of  the  soil  itself,  the  inge- 
nious expedient  was  hit  upon  to  force  the  water  from  within 
the  pile,  by  pumping  air  into  it,  and  thus  enable  workmen 
to  descend  to  the  bottom  and  remove  the  soil  or  other  ob- 
struction to  the  descent.  The  plan  devised  for  this  purpose 
was  to  fit  another  air-tight  iron  cylindrical  vessel  C  to  the 
top  of  the  pile,  of  sufficient  diameter  and  height  to  hold 
several  w^orkmen,  and  a  wdndlass  W,  arranged  with  an  end- 
less rope  and  buckets  for  raising  the  excavated  soil  into  the 
chamber  C. 

The  chamber,  which  has  received  the  name  of  an  air-lock 
from  its  functions,  w^as  provided  with  an  upper  man-hole  M  at 
top  for  entering  the  lock,  and  one  N  in  the  bottom  for  enter- 
ing the  pile.  Each  man-hole  had  tw^o  air-tight  valves,  one 
opening  outwards,  the  other  inwards.  Two  tubes,  C  and  D, 
wdth  stop-cocks,  furnished  an  air-passage  betw^een  the  air  of 
the  pile  and  that  of  the  lock,  and  betw^een  the  latter  and  the 
external  air.  A  syphon-shaped  water-discharge  tube  B,  wdth 
a  stoj^-cock,  leads  from  below  the  level  of  the  inner  water 
surface  through  the  bottom  and  side  of  the  lock. 

The  operation  of  sinking  the  pile  by  first  exhausting  the 
air  from  the  exhaust  chamber  D,  was  the  same  in  this  case 
as  in  the  preceding ;  the  upper  valves  of  either  man-hole  be- 
ing closed,  and  all  communication  between  the  external  air 
and  the  interior  of  the  pile  being  cut  off  by  means  of  the 
stop-cocks. 

Wlien  it  became  necessary  to  descend  to  the  bottom  of  the 
pile,  to  remove  the  soil  or  any  obstruction,  the  lower  valve  of 
the  lower  man-hole,  wdth  the  tube  C,  w^ere  closed ;  the  dis- 
charge tube,  E,  left  open ;  and  the  air  forced  into  the  pile, 
by  the  pumps,  through  the  tube  A ;  the  increased  pressure 
upon  the  water  surface  caused  the  water  to  rise  in  the  tube 
B,  and  flow  out  at  the  other  end. 

When  all  the  water  was  discharged  in  this  w^ay,  the  lower 
valve  of  the  upper  man-hole,  and  tubes  A,  B,  and  D  were 
closed ;  the  tube  C  was  then  opened,  through  w^liich  the  con- 
densed air  in  the  pile  flowed  into  the  lock,  until  the  density 
of  the  air  in  it  and  in  the  pile  became  the  same ;  the  lower 
valve  of  the  low^er  man-hole  was  then  opened,  to  allow  the 


FOTJOTDATIONS  OF  STRUCTURES. 


223 


workmen  to  descend,  and  the  excavated  soil  to  be  raised  into 
the  lock-chamber. 

To  take  the  excavated  material  out  of  the  lock,  the  lower 
man-hole  under  valve  and  the  tube  C  are  closed,  and  the  tube 
D  opened  ;  the  condensed  air  of  the  lock  flows  out,  and  the 
upper  man-hole  lower  valve  is  opened. 

In  some  of  the  more  recent  cases  of  the  application  of 
pneumatic  piles,  the  exhaust-chamber  and  the  discharge 
water-pipe  have  been  suppressed  ;  condensed  air  being  alone 
used,  both  to  force  the  internal  water  out  through  the  open 
bottom  of  the  pile,  to  allow  the  workmen  to  excavate  within, 
and  also  to  produce  a  scour  below  the  lower  end,  from  the 
rush  of  the  water  back  into  the  pile,  by  allowing  the  con- 
densed air  to  escape  rapidly  from  it.  For  this  purpose 
a  tube  leads  from  the  air-pumps  through  the  side  and  bottom 
of  the  air-lock,  into  the  pile,  to  supply  the  c;ompressed  air. 
Another  pipe  with  a  stop-cock  leads  through  the  side  and 
bottom  of  the  lock,  from  the  external  air  to  the  interior  of 
Jjie  pile,  through  which  the  condensed  air  in  tlie  pile  can  be 
discharged,  the  upper  and  lower  man-holes  have  each  an 
under  valve.  Two  equilibrium-tubes  with  stop-cocks,  one 
forming  a  connection  between  the  interior  of  the  pile  and  the 
air-lock,  the  other  leading  through  the  side  of  the  lock  to  the 
external  air,  furnish  the  means  of  bringing  the  air  of  the 
lock  to  the  same  density  as  that  within  the  pile,  or  that  of 
the  atmosphere. 

To  force  out  the  water,  the  lower  man-hole,  the  condensed 
air  discharge  pipe,  and  the  condensed  air  equilibrium-tube 
are  closed,  and  the  air  then  forced  into  the  -pile  by  the 
pumps. 

To  excavate  the  internal  soil,  the  workmen  enter  the  lock, 
close  the  upper  man-hole  and  the  upper  equilibrium-tube, 
and  open  the  lower  equilibrium-tube.  This  establishes  an 
equilibrium  between  the  air  of  the  lock  and  that  of  the  pile, 
and  the  workmen  can  then  descend  into  the  pile  and  exca- 
vate the  soil. 

To  remove  the  excavated  soil  which  has  been  raised  into 
the  lock,  the  lower  man-hole  and  lower  equilibrium-tube  are 
closed,  and  the  upper  equilibrium-tube  opened,  which  estab- 
lishes an  equilibrium  between  the  air  of  the  lock  and  that  of 
the  atmosphere.  The  upper  man-hole  then  being  opened, 
the  material  in  the  lock  can  be  carried  out. 

T^  produce  a  scour  under  the  pile  to  allow  it  to  sink,  the 
workmen  leave  the  pile  and  lock ;  the  condensed  air  dis- 
charge-pipe is  then  opened,  and  by  the  rush  of  the  water 


224 


CIVIL  ENGINEEEmG. 


into  the  pile  all  obstruction  to  the  movement  of  the  pile  is 
removed  from  its  lower  end. 

458.  Double  Air-Locks.  In  some  of  the  more  recent  ap- 
plications of  condensed  air  in  Europe,  air-locks  in  paii-s  have 
been  used  to  save  time. 


Fig.  56, — Longitudinal  Section  of  pile  A, 
bell  or  working-chamber  B,  and  air-locks 
C,  D,  used  at  the  bridge  at  Szegedin, 
over  the  river  Theiss,  Hungary. 

A,  water  discharge-pipe. 

B,  equilibrium  tubes  of  air-lock. 

C,  elevation  of  air-lock. 

D,  longitudinal  section  of  air-lock. 
M,  hoisting-gear  in  the  beU. 

N,  hoisting-gear  for  air-lock. 

W,  counterpoise  to  compressed  air. 


The  arrangements  in  this  case  (fig.  56)  consist  of  a  work- 
ing chamber,  B,  termed  the  hell^  which  is  a  large  air-tight 
iron  cylindrical  vessel  fastened  to  the  head  of  the  pile,  in 
which  there  is  sufficient  room  for  a  hoisting  apparatus,  M, 
and  several  workmen,  to  raise  the  excavated  soil  to  the  level 
of  the  air-locks;  of  two  small  air-locks,  D  and  C,  which  are 
inserted  into  the  bell  about  two-thirds  of  their  length  ;  of  a 
syphon-shaped  water  discharge-pipe  A ;  and  of  a  windlass  N 
to  raise  the  excavated  soil  out  of  the  locks. 

Each  lock  has  a  man-hole,  with  an  undervalve  on  top,  for 


FOUNDATIONS  OF  STEUOTUHES. 


225 


entering  cnt;  lock,  and  a  vertical  door  on  the  side  for  enter- 
ing the'bell.  Each  is  provided  with  two  sets  of  eqnilibriuni 
valves,  so  arranged  that  they  can  be  opened  by  a  person  from 
within  the  bell  or  the  lock,  to  establish  an  equilibrinm 
between  the  air  in  them  ;  or  from  the  outside  of  the  lock,  or 
the  inside,  to  establish  an  equilibrium  between  the  external 
air  and  that  of  the  lock. 

The  air  in  the  pile  is  condensed  by  air-pumps  in  the  usual 
way. 

The  hoisting-engine  in  the  bell  has  its  gearing  so  arranged 
that  the  filled  buckets  can  be  delivered  alternately  into  the 
locks,  and  from  there  be  taken  by  the  gearing  of  the  windlass 
above.  In  the  example  represented  by  Fig.  56,  a  weight,  W, 
formed  of  cast-iron  bars,  resting  on  brackets  cast  on  the  out- 
side of  the  bell,  forms  a  counter-pressure  to  the  interior  con- 
densed air. 

The  pile  is  sunk  by  opening  a  condensed  air-pipe  leading  to 
the  external  air,  the  lower  portions  of  water  discharge-pipe 
having  been  removed,  and,  with  the  tools  used  in  excavating, 
placed  within  the  bell. 

The  descent  of  the  pile  at  each  discharge  of  the  condensed 
air  depends  upon  the  nature  of  the  strata  met  with.  In  very 
compact  clay  the  descent  will,  in  some  instances,  be  only  a 
few  inches  in  several  discharges ;  while  in  sandy  or  gravelly 
strata  it  will  descend  as  much,  at  times,  as  twelve  or  more 
feet.  This  is  owing  to  the  difference  between  the  effect  of 
the  scour,  and  the  resistance  offered  by  the  friction  on  the- 
exterior  surface  of  the  pile.  The  resistances  in  sand  and' 
gravel  being  much  less  than  in  stiff  clay.  It  has  been  found,, 
in  some  cases,  that  two  or  three  feet  of  a  compact  clay  soil 
left  within  the  piles  at  the  bottom  would  prevent  the  scour 
and  the  further  descent  of  the  pile  when  the  condensed  air 
was  discharged. 

The  piles  are  placed  in  position  by  a  suitable  hoisting- 
gearing  raised  upon  a  strong  scaffolding ;  and  in  their  descent 
are  kept  in  a  vertical  position  by  guides  placed  in  connection 
with  the  scaffolding.  Great  precautions  have  to  be  taken  in 
managing  the  descent  of  the  pile,  when  it  is  approaching  the 
depth  to  which  it  is  wished  to  sink  it,  so  as  to  keep  the  top^ 
surface  of  each  on  the  same  level. 

In  the  first  applications  of  pneumatic  piles,  cast-iron  cylin- 
ders of  small  diameters  were  used ;  as  many  being  sunk  as 
the  resistance  of  the  substratum  upon  which  they  rested  re-» 
quired  to  support  the  base  of  the  superstructure.  Subse- 
quently the  diameters  of  the  cylinders  were  enlarged,  to 


226 


CIVIL  EXGINEEEING. 


enable  tlie  soil  to  be  excavated  from  the  interior,  and  be 
replaced  with  hydraulic  concrete.  In  some  instances  the 
concrete  simply  rested  upon  the  bottom  of  the  excavation. 
In  others,  wooden  piles  were  driven  within  the  cylinder  some 
distance  below  its  lower  end,  and  the  concrete  thrown  in  to 
rest  upon  the  heads  of  the  piles. 

Haiiem  Bridge. — In  the  Harlem  Bridge  the  piles  were  six 
feet  in  diameter,  and  cast  in  lengths  of  ten  feet.  The  air- 
lock was  of  the  same  diameter  as  the  piles,  and  six  feet  high ; 
the  valves  or  man-holes  twenty  inches  in  diameter.  The  most 
noticeable  feature  in  this  part  of  the  structure,  is  the  expedient 
of  using  an  underpinning  of  plank  and  concrete,  to  obtain  a 
wider  spread  of  the  foundation  bed,  and  a  greater  bearing 
surface  for  the  superstructure  to  rest  on.  For  this  purpose, 
plank  five  feet  long,  three  inches  wide,  and  one  inch  and  a 
quarter  thick  (Fig.  55)  w^ere  forced  under  the  bottom  of  the 
pile,  in  sections  of  three  feet  wide  on  opposite  sides,  and  in 
an  inclined  direction,  so  as  to  gain  an  additional  spread  of 
foundation  base  of  two  feet  around  and  beyond  the  pile. 
These  formed  a  temporary  roofing,  from  beneath  which  the 
soil  was  rapidly  removed,  and  the  excavated  space  filled  in 
with  concrete.  Finding  great  inconvenience  in  this  process, 
from  the  rapidity  with  which  the  water  and  sand  came  in  ou 
the  sides,  an  additional  condensation  was  given  to  the  com- 
pressed air  of  six  to  ten  feet  extra  v*^ater  pressure ;  this  was 
found  to  counteract  the  external  pressure,  so  as  to  allow  the 
excavations  to  be  carried  on  with  facility. 

The  refuse  gas-pipes  w^hich  were  used  to  convey  the  com- 
pressed air  down  between  the  bottom  of  the  concrete  and  the 
underlying  soil,  as  well  as  giving  it  a  passage  between  the 
outside  of  the  pipes  and  the  body  of  the  concrete,  were  dis- 
tributed through  the  concrete  about  a  foot  apart. 

The  bottom  of  the  foundation  in  this  example  was  thirty 
feet  below  the  surface  of  the  river-bed,  and  fifty  below 
tide. 

An  opinion  has  obtained,  from  the  condition  in  which  the 
hydraulic  concrete  was  found  in  a  pile  accidentally  fractured, 
in  which  it  had  lain  for  some  time,  that  this  material  did  not 
harden  when  subjected  to  the  great  pressure  of  the  water 
from  the  bottom.  A  remedy,  it  is  stated,  has  been  found  for 
this  by  using  a  portion  of  fragments  of  a  porous  brick  in  a 
dry  state  instead  of  stone,  in  the  composition  of  the  con- 
crete, as  was  done  in  the  case  of  the  piei-s  of  the  bridge  at 
Szegedin,  in  Hungary  ;  and  by  inserting  in  the  body  of  the 
concrete  half -inch  gas-pipes,  through  which  the  compressed 


PNEUMATIC  riLES. 


227 


air  was  diffused  throughout  the  mass,  as  practised  at  the 
Harlem  Bridge  by  Mr.  McAlpine. 

Bridge  over  the  Theism. — The  soil  below  the  bed  of  the 
river  Theiss,  at  Szegedin,  is  alluvial,  and  fomid  in  alternate 
strata  of  compact  clay  and  sand  to  an  indefinite  depth.  The 
current  thronghout  its  course  is  sluggish,  having  a  surface  velo- 
city at  Szegedin,  during  the  highest  stage  of  the  waters,  of  from 
three  to  three  and  a  half  feet.  The  rise  and  fall  of  tlie  water 
are  both  very  gradual ;  the  highest  stage  being  about  twenty- 
six  feet,  and  the  mean  level  about  sixteen  feet.  The  arched 
ribs  and  other  superstructure  of  the  bridge  were  of  wrought- 
iron  plates.  Each  pier  was  formed  of  two  piles,  or  columns, 
filled  with  beton,  as  above  described ;  and  each  suj^porting 
one  track  of  the  railroad.  They  were  cast  in  lengths  of  six 
feet,  and  ten  feet  in  diameter,  and  one  inch  and  one-tenth  in 
thickness.  The  piles  were  sunk  to  the  depth  of  about  thirty 
feet  below  the  surface  of  the  bed  ;  and  about  forty  feet  below 
the  ordinary  low-water  level.  Their  height  corresponded  to 
the  highest  water  level,  or  nearly  thirty-three  feet  above  the 
presumed  scour  of  the  bed. 

The  interior  excavation  of  the  soil  was  carried  down  to  the 
first  joint,  or  six  feet  from  the  bottom  of  the  column.  To 
compress  the  soil  below  the  column  to  sustain  better  the  su- 
perincumbent weight,  twelve  piles  of  pine  were  driven  within 
the  column  to  a  depth  of  twenty  feet  below  the  bottom. 

The  air-locks  were  each  about  six  feet  six  inches  in  height, 
and  two  feet  nine  inches  in  diameter. 

To  provide  against  the  scour  of  the  current,  the  entire  pier 
w^as  enclosed  by  a  row  of  square  sheeting-piles,  driven  to  the 
level  of  the  bottom  of  the  columns,  and  about  two  feet  from 
them.  The  space  between  these  piles  and  columns,  to  the 
depth  of  ten  feet  below  the  bed  level,  was  filled  with  hydraulic 
concrete ;  and  the  piles  were  surrounded  by  loose  stone  with 
a  spread  of  about  ten  feet  from  the  piles. 

As  large  quantities  of  hydraulic  concrete  are  required  for 
filling  the  piles,  the  method  pursued  in  Germany,  and  as 
practised  at  the  bridge  at  Szegedin,  for  mixing  the  mortar  and 
fragments  of  brick  or  stone,  commends  itself  for  its  economy, 
and  the  thoroughness  with  which  the  materials  are  incor- 
porated. A  wooden  cylinder  about  twelve  feet  long,  and  four 
feet  diameter,  made  and  hooped  like  a  barrel,  and  lined  with 
sheet-iron,  placed  in  an  inclined  position  of  ^3-  to  the  horizon, 
was  made  to  revolve  by  a  band  set  in  motion  by  a  steam-en- 
gine, from  fifteen  to  twenty  revolutions  in  a  minute.  The 
cylinder  was  fed  by  a  hopper  at  the  upper  end,  into  which 


228 


CIVIL  ENGINEERING. 


the  materials  were  thrown,  and  they  were  discharged  thor- 
oughly mixed  and  ready  for  use  into  wheelbarrows  at  the 
lower  end.  It  is  stated  that  this  simple  machine  delivered 
from  280  to  350  cubic  feet  in  ten  hours. 

The  concrete  is  usually  thrown  down  into  the  pile  from  the 
bell  or  lock.  At  the  bridge  at  Szegedin  the  double  locks 
were,  alternately,  nearly  filled  with  the  concrete,  and  it  was 
raked  out  from  them  and  thrown  into  the  pile ;  care  being 
taken  to  work  it  in  well  by  hand,  around  the  flanges  and 
joints. 


Fig.  57. 


Fig.  57.— Longitudinal  section  of  the  cais- 
son and  masonry  of  a  pier  of  a  railroad 
bridge  over  the  Scarff,  at  L'Orient, 
France. 

Fig.  58.— J'lan. 

A,  working-chamber  for  excavating  soil. 

B,  interior  elevation  of  caisson. 

C,  C,  elevation  of  the  bells  containing  the 

double  air-locks. 

D,  D,  cylinders  for  communication  between 
bells  and  working-chamber. 


Bridg-e  over  the  Savannah  River  on  the  line  of  the 
Charleston  and  Savannah  Rail  Road.  The  air-locks  on 
these  piles  were  similar  to  the  Harlem  plan.  Light  was 
admitted  into  the  air-lock  by  means  of  large  bulls-eye  glasses, 
and  thence  into  the  body  of  the  pile  in  the  same  way,  but 
this  mode  w^as  found  to  be  worthless,  on  account  of  the  mud 
in  the  bottom  of  the  air-lock  which  covered  the  glass.  The 
engineer  employed  a  secondary  small  air  lock  so  that  the 
material  which  was  brought  into  the  main  one  could  be  dis- 
charged at  any  time,  and  thus  the  work  go  on  with  less 
interruption,  and  the  bulls-eyes  became  more  serviceable. 
With  the  secondary  air-lock  the  work  progressed  more  rapidly ; 
the  ratio  for  a  given  amount  of  work  being 
Time  hy  old  air-lock  _14 
Time  hy  new  air-lock  5 

By  a  fortunate  discovery  the  engineer  discovered  that  the 
pressure  of  the  air  in  the  pile  was  sufficient  to  force  sand  from 


PNEUMATIC  CAISSONS. 


229 


the  bottom  of  the  pile  through  a  vertical  pipe  to  a  height 
above  the  surface  of  the  water  outside  the  works.  A  sort  of 
telescopic  tube  was  attached  to  the  lower  end  of  a  pipe  so 
that  it  could  be  easily  moved  downward  as  the  excavation 
progressed.  This  greatly  facilitated  the  progress  of  the  work, 
lor  it  w^as  found  that  to  do  a  given  amount  of  work  the  ratio 
was 

Time  hy  old  air-lock ...  14 

Time  hy  blowing  out  sand  ~  -J  ~ 
This  mode  of  excavation  has  been  adopted  to  some  extent  in 
the  caissons  of  the  East  River  Bridge.  This  process  also 
secures  thorough  ventilation.  The  same  plan  has  also  been 
used  in  the  Omaha  Bridge  and  Leavenworth  Bridge  with 
equally  good  results. 

It  is  sometimes  very  difficult  to  keep  the  tubes  vertical. 
Wlien  they  begin  to  incline  efforts  should  be  made  immedi- 
ately to  bring  them  to  an  erect  position.  In  some  cases 
wedges  or  blocks  placed  under  the  low^er  side  and  suddenly 
relieving  the  pressure  will  correct  the  evil.  An  ingenious 
mode  was  adopted  by  the  engineer  of  the  Omaha  Bridge.  Tie 
bored  several  holes  through  the  tubes  on  the  upper  side, 
through  w^hich  the  compressed  air  escaped  and  thus  disturbed 
the  soil  and  relieved  the  pressure  on  that  side  so  that  it  would 
sink  faster.  Strong  levers  have  been  used  to  pull  on  the  top 
whilst  the  tube  was  sinking,  but  not  with  very  marked  re- 
sults. In  at  least  one  very  obstinate  case,  in  which  the  holes 
on  the  uj)per  side,  combined  with  the  action  of  a  strong  lever, 
did  not  alone  effect  the  desired  result,  a  ram  was  used  in 
combination  with  the  other  devices  and  the  erect  position  was 
quickly  secured.  The  jar  produced  by  the  ram  whilst  the 
tube  was  sinking  seemed  to  give  great  effect  to  the  other 
devices. 

Gen.  W.  S.  Smith,  who  had  charge  of  the  construction  of 
the  foundations  of  the  Omaha  and  Leavenworth  Bridges,  is  of 
opinion  that  a  pneumatic  caisson,  surmounted  by  masonry,  is 
cheaper  and  better  than  pneumatic  pile  piers,  but  it  is  evident 
that  circumstances  may  often  determine  which  is  preferable 
in  any  particular  case. 

459.  Pneumatic  Caissons.  The  application  of  compressed 
air  for  laying  foundations  has  been  further  extended  in  some 
of  the  railroad  bridges  recently  constructed  in  Europe  ;  by 
using  wrought-iron  caissons  of  sufficient  dimensions  to  serve 
as  an  envelope,  or  jacket,  for  the  masonry  of  an  entire  pier ; ' 
and  gradually  sinking  the  whole  to  the  requisite  depth,  by 
excavating  the  soil  within  the  pier  to  the  desired  level. 


230 


OmL  ENGINEERING. 


The  caissons  (Figs.  57,  58)  for  this  purpose  were  divided 
into  two  compartments. 

The  lower  A  (Fig.  57),  which  served  as  a  chamber  for  the 
workmen,  for  excavating  the  soil,  was  strongly  roofed  at  top, 
with  iron  bars  and  iron  sheeting,  to  bear  the  weight  of  the 
masonry  that  rested  upon  it ;  and  was  securely  buttressed  on 
the  sides  to  resist  the  inward  pressure  of  the  soil  on  the  out- 
side. The  upper  chamber,  B,  served  as  an  ordinary  caisson, 
fitting  closely  to  the  masonry  on  the  sides,  and  rising  suffi- 
ciently above  it  to  exclude  the  water  during  the  construction 
of  the  masonry :  the  body  of  which,  composed  of  beton  with 
a  facing  of  stone,  was  gradually  raised  as  the  caisson  was  sunk 
through  the  earth  overlying  a  bed  of  rock  upon  which  the 
pier  was  finally  to  rest. 

The  working  chamber  A  was  connected  with  two  bells  C, 
C,  by  two  vertical  iron  cylinders  D,  T>  (Fig.  57),  for  each 
bell ;  these  cylinders  serving  as  a  communication  between  the 
working-chamber  and  bells,  for  the  passage  of  the  workmen 
from  one  to  the  other,  for  raising  the  excavated  soil,  and  as  a 
passage  for  the  compressed  air  forced  in  by  the  air-pnmps. 

Each  bell  contained  two  air-locks  for  communicating  be- 
tween it  and  the  exterior ;  and  a  hoisting-gearing  for  the 
excavated  soil;  the  filled  buckets  ascending  through  one 
cylinder,  and  the  empty  ones  descending  through  the  other. 

The  lower  chamber,  the  bottom  of  which  was  open,  w^as 
kept  filled  with  compressed  air  of  sufficient  density  to  exclude 
the  water,  and  enable  the  workmen  to  excavate  the  soil. 

The  caisson  was  gradually  sunk,  by  the  weight  of  the 
superincumbent  mass,  as  the  soil  below  was  removed. 

So  soon  as  the  rock-bed  was  reached,  the  surface  was 
thoroughly  cleaned  off,  and  levelled  under  the  edges  of  the 
bottom  of  the  caisson,  and  the  chamber  A  was  gradually 
filled  in  with  masonry  closely  up  to  its  roof.  Finally  the 
cylinders  D  were  removed,  and  the  wells  occupied  by  them 
in  the  body  of  the  pier,  filled  with  beton. 

As  a  matter  of  interest,  on  the  subject  of  laying  founda- 
tions by  means  of  pneumatic  piles  and  caissons,  a  few  addi- 
tional facts  in  coimection  with  the  examples  above  cited  will 
not  be  out  of  place  here. 

Bridge  over  the  ScorfT  In  the  example  of  the  bridge  at 
L'Orient  over  the  Scorff,  the  river-bed  is  a  stratum  of  nuid, 
forty-six  feet  in  depth,  resting  upon  a  surface  of  hard  schis- 
toze  rock  more  or  less  inclined  and  uneven.  The  level  of 
mean  tide  is  about  sixty  feet  above  the  rock  surface  ;  that  of 
the  highest  tide  seventy  feet. 


PNEUMATIC  CAISSONS. 


231 


The  caissons  used  in  this  example  were  designed  for  the 
piers  of  a  stone  bridge,  and  were  about  forty  feet  long  and 
twelve  feet  broad.  The  bells,  or  upper  working  chambers, 
were  ten  feet  high  and  eight  feet  in  diameter ;  the  lower 
working-chamber  ten  feet  high ;  and  the  cylinders,  for  com- 
munication between  them,  two  feet  and  a  half  in  diameter. 

The  caissons  were  built  of  sheet-iron,  in  zones  decreasing  in 
thickness  from  the  top  to  the  bottom  ;  but  not  having  been 
buttressed  within  against  the  pressure  of  the  water,  as  the 
lower  working-chamber  was,  they  yielded  and  got  out  of 
shape. 

In  a  subsequent  structure  of  nearly  the  same  dimensions, 
for  a  railroad  bridge  at  JSTantes,  the  same  failure  took  place, 
and  precautions  were  then  taken  against  it  by  the  insertion  of 
cross-stays,  which  were  removed  as  the  masonry  was  carried 
up.  In  the  caissons  used  in  this  case,  the  bells  and  air-locks 
were  made  larger.  Each  air-lock  had  three  separate  com- 
partments ;  one  for  the  passage  of  the  workmen  which  could 
contain  four  men  ;  one  for  the  barrows  by  which  the  excavated 
soil  was  removed,  and  one  for  the  concrete  to  till  up  the 
lower  working  chamber,  when  the  excavation  was  completed. 

St.  Louis  Bridge.  The  caissons  for  the  two  piers  of  this 
bridge  differ  in  no  material  respect,  so  that  a  description  of 
one  will  equally  apply  to  the  other.  The  air-chambers  are 
nine  feet  high,  the  sides  being  formed  of  f-inch  plate  iron  in 
the  larger,  and  f-inch  in  the  smaller.  The  air-chamber  is 
simply  a  huge  diving-bell  of  the  full  size  of  the  pier.  The 
iron  plates  K,  K  (Fig.  59),  forming  its  roof,  are  of  -J-inch 
thickness.  Transversely  over  this  and  riveted  firmly  to  it 
are  thirteen  iron  girders  L,  at  intervals  of  five  and  a  half  feet. 
Beneath  the  roof  two  massive  timber  girders  C,  C  (Figs.  59 
and  60),  in  an  opposite  direction  to  the  iron  ones,  divide  the 
air-chamber  into  three  nearly  equal  parts.  Communication 
between  the  three  divisions  is  had  through  openings  made  for 
this  purpose  in  the  girders.  These  timber  girders  are 
intended  to  rest  on  the  sand  and  support  the  roof  from  below. 
The  sides  of  the  air-chambers  are  strongly  braced  with  plate 
iron  brackets  O  O,  stiffened  with  angle  iron.  Between  the 
brackets  is  placed  all  around  the  chamber  a  course  of  strong 
timbers,  the  bottom  of  which  is  level  with  that  of  the  girders, 
intended  to  rest  on  the  sand  and  assist  in  supporting  the 
weight.  The  support  given  by  the  timbers,  together  with  the 
buoyant  power  of  the  compressed  air  in  the  chamber  and  the 
friction  of  the  sand  on  the  sides,  is  the  only  means  relied  on 
to  sustain  the  pier  in  its  gradual  descent  to  the  rock. 


232 


CIVIL  ENGINEERING. 


Fig.  59. 


Fig.  59— Represents  the  plan  of  the  caisson  of  the  East  pier  of  the  Illinois  and  St.  Louis  Bridge. 
Fig.  (50  represents  transverse  section  of  the  same.  A.  air-locks.  B,  air-chamber.  C.  timber 
girders.  D,  discharge  of  sand.  E,  sand-pumps.  F,  main  entrance  shaft.  G,  side  shafts.  H, 
iron  sides.   I,  bracing  for  H.   K,  iron  deck  or  roof.   L,  iron  girders.    0,  sti-engthening  girders. 


PNEUMATIC  CAISSONS. 


233 


The  air-locks  A  A,  heretofore  as  a  rule  placed  above  the 
surface  of  the  water,  are  located  within  the  roof  of  the  air- 
chamber,  and  access  is  had  to  them  through  brick  wells  F,  G, 
thus  avoiding  the  inconvenience  and  delay  of  adding  new 
joints  under  the  locks. 

The  sand-pumps  E  are  placed  on  the  roof  of  the  chamber, 
their  suction  pipes  extending  through  the  chamber  to  the 
sand.    The  action  of  these  pumps  is  very  simple.    A  stream 
of  water  is  forced  down  the  pipe  B,  (Fig.  61),  and  discharged 
near  the  sand  into  the  pipe  A,  through 
Fig.  61.  ^j^Q  annular  jet  C.     The  jet  creates 

a  vacuum  below  it,  by  which  the  sand 
is  drawn  into  the  second  pipe,  the  lower 
end  of  which  is  in  the  sand,  and  the 
force  of  the  jet  carries  it  up  to  the 
mouth  of  the  pump  so  soon  as  it  passes 
C. 

The  abutments  at  the  east  end  of  the 
bridge  (Figs.  Ql  a  and  61  h)  differed  in 
some  of  the  details  of  their  construction 
from  the  piers. 


Fig.  61  a. 


Fig.  61,  represents  a  vertical  sec- 
tion of  a  sand-pump. 

A,  pump  barrel. 

B,  injection  pipe. 

C,  annular  jet. 


Fig.  61  a,  is  a  part  plan  and  part  section  of  the  east  abut- 
ment of  the  St.  Louis  Bridge.  Fig.  61  b,  is  a  vertical  section 
of  the  same. 

I,  is  the  main  shaft. 

KK.  the  side  shafts. 

MM,  iron  girders. 

00,  the  air-locks. 

PP,  the  air-chamber. 

QQ,  the  timber  girders. 

KR,  the  timber  deck. 

SS,  the  iron  sheeting. 

TT,  the  timber  sides  of  the  caisson. 


234 


CIVIL  ENGINEEKING. 


The  main  shaft  had  two  air-hjcks  at  the  lower  end,  each  8 
feet  in  diameter,  having  about  four  times  the  capacity  of  the 
one  nsed  in  the  piei'S.  There  were  also  two  other  shafts  and 
air-locks  wiiich  were  introduced  to  secure  additional  safety. 
This  caisson  w^as  probal)ly  sunk  to  a  greater  depth  than  any 
other  in  the  w^orld  by  the  pneumatic  process. 

It  was  sunk  to  the  native  rock,  which  was  136  feet  below 
high-water  mark,  and  94  below  the  extreme  low-water  mark. 
It  was  about  110  feet  below  the  surface  of  the  water  at  the 
time  it  was  completed.  It  was  extremely  hazardous  to  the 
health  aud  even  lives  of  workmen  to  be  kept  under  the  pres- 
sure of  over  three  atmospheres  for  a  long  time.  The  greatest 
security  was  found  in  changing  them  every  three  or  fouv 
hours. 

Candles  burned  very  readily  at  tliis  deptli  and  pressure. 
After  a  depth  of  about  SO  feet  was  reached,  the  candles  were 
inclosed  in  a  strong  glass  globe,  the  inside  of  which  connnuui- 
cated  with  one  of  the  shafts,  and  the  pressure  was  i-eguhited 
by  a  small  tube  passing  through  the  globe  and  containing  a 
check  valve.  In  this  way  the  candles  burned  in  an  atmos- 
phere whose  pressure  was  about  the  same  as  the  external  air. 
(See  London  Engineering^  1S70  and  1S71.) 

East  River  Bridge.  The  caisson  for  this  bridge  is  com- 
posed almost  wholly  of  wood.  The  air-chamber  (Fig.  ^rl)  is 
nine  feet  six  inches  high,  the  roof  being  made  of  fifteen 
courses  of  timbers,  one  foot  thick,  the  lower  five  (A)  being 


CAISSON  OF  EAST  EIVER  BRIDGE. 


235 


laid  solid,  the  upper  ten  (C)  crossing  in  alternate  layers,  and 
placed  about  a  foot  apart,  the  sj^aces  between  the  timbers  being 
tilled  with  concrete.  The  sides  (B)  of  the  air-chamber  are  Y 
shape,  made  very  solid,  nine  feet  thick  at  top,  and  eight  inches 
at  the  bottom,  which  is  heavily  shod  with  iron.  Between 


Fig.  62  represents  a  vertical  section  of  the  Brooklyn  Caisson  of  the  East  River  Bridge. 

A,  lower  timber  courses  of  roof,  laid  solid. 

B,  timber  sides  of  air-chamber. 

C,  upper  timber  courses  of  roof,  laid  crosswise,  and  spaces  filled  in  with  concrete. 

D,  masonry  of  pier. 

E,  dam  to  prevent  water  from  reaching  shafts. 

F,  air-shaft  and  lock. 

G,  supply  shaft, 

E,  excavation  shaft. 

I,  heavy  timber  partitions. 

K,  air-chamber. 

the  fourth  and  fifth  courses  of  the  roof  is  laid  a  sheet  of  tin, 
which  is  continued  down  underneath  the  outside  sheathing. 
The  air-chamber  is  divided  into  six  compartments  by  heavy 
timber  girders.  The  shafts  through  which  the  heavy  material 
is  raised  extend  below^  the  level  of  the  excavation  at  the 
bottom,  and  are  constantly  open ;  but  the  compressed  air  is 
prevented  from  escaping  by  a  column  of  water,  which  is 
maintained  at  nearly  the  same  height  as  the  water  in  the  river 
by  the  pressure  of  the  compressed  air.  If  the  pressure  of  the 
air  should  be  made  to  greatly  exceed  that  at  which  it  is  ordi- 
narily maintained,  it  would  blow  all  the  water  out  of  the  shaft 


236 


CIVIL  ENGINEERING. 


and  the  air  would  entirely  escape,  but  every  necessary  pre- 
caution was  used  to  keep  a  prpper  pressure  of  the  air.  An  ac-. 
cident  of  this  kind  once  took  place  in  the  Brooklyn  caisson. 


YII. 

CONSTRUCTION  OF  MASONRY. 

460.  Under  this  head  will  be  comprised  whatever  relates 
to  the  manner  of  determining  the  forms  and  dimensions  of 
the  most  important  elementary  components  of  structures  of 
masonry,  together  with  the  practical  details  of  their  construc- 
tion. 

461.  Foundation  Courses.  As  the  object  of  the  founda- 
tions is  to  give  greater  stability  to  the  structure  by  diffusing 
its  weight  over  a  broad  surface,  their  breadth,  or  sjprea/1^ 
should  be  proportioned  both  to  the  weight  of  the  structure 
and  to  the  resistance  offered  by  the  subsoil.  In  a  perfectly 
unyielding  soil,  like  hard  rock,  there  will  be  no  increase  of 
stability  by  augmenting  the  base  of  the  structure  beyond 
what  is  strictly  necessary  for  stability  in  a  lateral  direction  ; 
whereas  in  a  very  compressible  soil,  like  soft  mad,  it  would 
be  necessary  to  make  the  base  of  the  foundation  very  broad, 
60  that  by  diffusing  the  weight  over  a  great  surface,  the  sub- 
soil may  offer  sufficient  resistance,  and  any  unequal  settling 
be  obviated. 

462.  The  thickness  of  the  foundation  course  will  depend 
on  the  spread ;  the  base  is  made  broader  than  the  top  for  mo- 
tives of  economy.    This  diminution  of  the  volume  (Fig.  63) 


Fig.  63— Section  of  foundation  courses  and  Buperstructure. 

A,  batter. 

B,  offsets. 

C,  superstructure. 


is  made  either  in  stops,  termed  offsets,  or  else  by  giving  a 
imiform  batter  from  the  base  to  the  top. 

When  the  foundation  has  to  resist  only  a  vertical  pressure, 


FOUNDATIONS. 


237 


an  equal  batter  is  given  to  it  on  each  side ;  but  if  it  has  to 
resist  also  a  lateral  effort,  the  spread  should  be  greater  on  the 
side  opposed  to  this  effort,  in  order  to  resist  its  tendency, 
which  would  be  to  cause  a  yielding  on  that  side. 

463.  The  bottom  course  of  the  foundations  is  usually 
formed  of  the  largest  sized  blocks,  roughly  dressed  off  with 
the  hammer ;  but  if  the  bed  is  compressible,  or  the  surfaces 
of  the  blocks  are  winding,  it  is  preferable  to  use  blocks  of  a 
small  size  for  the  bottom  course ;  because  these  small  blocks 
can  be  firmly  settled,  by  means  of  a  heavy  beetle,  into  close 
contact  with  the  bed,  which  cannot  be  done  with  large-sized 
blocks,  particularly  if  their  under  surface  is  not  perfectly 
plane.  The  next  course  above  the  bottom  one  should  be  of 
large  blocks,  to  bind  in  a  firm  manner  the  smaller  blocks  of 
the  bottom  course,  and  to  diffuse  the  weight  more  uniformly 
over  them. 

464.  When  a  foundation  for  a  structure  rests  on  isolated 
supports,  like  the  pillars,  or  columns  of  an  edifice,  an  in- 
verted  or  counter-arch,  (Fig.  64,)  should  connect  the  top 
course  of  the  foundation  under  the  base  of  each  isolated 
support,  so  that  the  pressure  on  any  two  adjacent  ones  may 
be  distributed  over  the  bed  of  the  foundation  in  the  interval 
between  them.  This  precaution  is  obviously  necessary  in 
compressible  soils. 


Fig.  64.— Section  of  vertical 
supports  on  reversed  arches. 

A,  reversed  arch. 

B,  vertical  supports. 

C,  bed  of  stone. 

The  reversed  arch  is  also  used  to  give  greater  breadth  to 
the  foundations  of  a  wall  with  counterforts,  and  in  cases 
where  an  upward  pressure  from  water,  or  a  semi-fluid  soil 
requires  to  be  counteracted.  In  the  former  case  the  reversed 
arches  are  turned  under  the  counterforts ;  in  the  latter  they 
form  the  points  of  support  of  the  walls  of  the  structure. 

465.  The  angles  of  the  foundations  should  be  formed  of 
the  most  massive  blocks.  The  courses  should  be  carried  up 
uniformly  throughout  the  foundation,  to  prevent  unequal 
settling  in  the  mass. 

The  stones  of  the  top  course  of  the  foundation  should  be 


238 


CIVIL  ENGINEERING. 


sufficiently  large  to  allow  the  course  of  the  superstructiire 
next  above  to  rest  on  the  exterior  stones  of  the  top  course. 

466.  Hydraulic  mortar  should  be  used  for  tlie  foundations, 
and  the  upper  courses  of  the  structure  should  not  be  com- 
menced until  the  mortar  has  partially  set  throughout  the 
entire  foundation. 

467.  Component  parts  of  Structures  of  Tvlasonry. 
These  may  be  divided  into  several  classes,  according  to  the 
efforts  they  sustain ;  their  forms  and  dimensions  depending  on 
these  efforts. 

1st.  Those  which  sustain  only  their  own  weight,  and  are  not 
liable  to  any  cross  strain  upon  the  blocks  of  which  they  are 
formed,  as  the  walls  of  enclosures. 

2d.  Those  which,  besides  their  own  weight,  sustain  a  verti- 
cal pressure  arising  fi^om  a  weight  borne  by  them,  as  the  walls 
of  edifices,  columns,  the  piers  of  arclies,  &c. 

3d.  Those  which  sustain  lateral  pressures,  and  cross  strains 
upon  the  blocks,  arising  from  the  action  of  the  earth,  water, 
frames  or  arches. 

4th.  Those  which  sustain  a  vertical  upward,  or  downward 
pressure,  and  a  cross  strain,  as  areas,  lintels,  &c. 

5th.  Those  which  transfer  the  pressure  they  directly  receive 
to  lateral  points  of  supports,  as  arches. 

468.  Walls  of  Enclosure.  AValls  for  these  purposes 
may  be  built  of  brick,  rubble,  or  dry  stone. 

Brick  walls  are  usually  built  vertically  upon  the  two  faces ; 
*   and  their  thickness  cannot  be  less  than  that  of  one  brick. 

Rubble  stone  walls  should  never  receive  a  thickness  less 
than  18  inches  when  the  two  faces  are  vertical.  Rondelet,  in 
his  work  VArt  de  Bdtir,  lays  down  a  rule  that  the  mean 
thickness  of  both  rubble  and  brick  walls  should  be  -^^of  their 
height ;  but  rubble  stone  walls  are  rarely  made  so  thin  as  this. 

Dry  stone  walls  should  not  receive  a  less  thickness  than  two 
feet.  When  their  height  exceeds  12  feet,  their  mean  thick- 
ness should  not  be  less  than  ^  of  the  height. 

Stone  walls  are  usually  built  with  sloping  faces.  The  batter 
should  not  be  greater,  when  the  stones  are  cemented  with 
mortar,  than  one  base  to  six  perpendicular,  in  order  that  the 
rain  may  run  rapidly  from  the  surface,  and  that  the  wall  be 
not  too  much  exposed  to  decay  from  the  germination  of  seeds 
which  may  lodge  in  the  joints. 

The  batter  is  arranged  either  by  building  the  wall  in  offsets 
from  top  to  bottom,  or  by  a  uniform  surface.  In  either  case, 
the  thickness  of  the  wall  at  top  should  not  be  less  than  from 
8  to  12  inches. 


EETAINING  WALLS. 


239 


When  a  wall  is  built  with  an  equal  batter  on  each  face,  and 
the  thickness  at  the  top  and  the  mean  thickness  are  fixed,  the 
hasG  of  the  wall,  or  its  thickness  at  the  bottom,  loill  he  found 
hy  suhtractirig  the  thickness  at  tojy  from  twice  the  mean  thick- 
7iess.  This  rule  evidently  makes  the  batter  of  the  wall  de- 
pend upon  the  two  preceding  dimensions. 

The  mean  thickness  of  long  walls  may  be  advantageously 
diminished  by  placing  counterforts,  or  buttresses,  upon  each 
face  at  equal  di&tances  along  the  line  of  the  wall.  These  are 
spurs  of  masonry  projecting  some  length  from  the  wall,  and 
are  firmly  connected  with  it  by  a  suitable  bond.  The  horizon- 
tal section  of  the  counterforts  may  be  rectangular ;  their 
lieight  should  be  the  same  as  that  of  the  wall. 

469.  Vertical  Supports.  These  consist  of  walls,  columns, 
or  pillars, '  according  to  circumstances.  The  dimensions  of 
the  courses  of  masonry  which  compose  the  supports  should  be 
regulated  by  the  weight  borne.  If,  as  in  the  walls  of  edifices, 
the  resultant  of  the  efforts  sustained  by  tjie  wall  should  not 
be  vertical,  it  must  not  intersect  the  base  of  the  wall  so  near 
the  outer  edge,  that  the  stone  forming  the  lowest  course  would 
be  in  danger  of  being  crushed. 

Cross  walls  between  the  exterior  walls,  as  the  partition 
walls  of  edifices,  should  be  regarded  as  counterforts  which 
strengthen  the  main  walls. 

470.  Areas.  The  term  area  is  applied  to  a  mass  of 
masonry,  usually  of  a  uniform  thickness,  laid  over  the  ground 
enclosed  by  the  foundations  of  walls.  It  seldom  happens  that 
areas  have  an  upward  pressure  to  sustain.  Whenever  this 
occurs,  as  in  the  case  of  the  bottoms  of  cellars  in  communica- 
tion with  a  head  of  water  which  causes  an  upward  pressure, 
the  thickness  and  arrangement  of  the  area  should  be  regulated 
to  resist  this  pressure.  When  the  pressure  is  considerable,  an 
area  of  uniform  thickness  may  not  be  sufficiently  strong  to  en- 
sure safety ;  in  this  case  an  inverted  arch  must  be  used.  The 
foundation  of  the  Capitol  building  at  Albany,  IS".  Y.,  rests  on 
an  immense  area,  which  is  formed  of  successive  la^^ers  of 
broken  stone  and  concrete,  making  an  area  of  several  feet  in 
thickness.  The  first  stones  of  the  piers  are  very  large  and 
fiat  and  nearly  cover  the  whole  area  so  that  there  is  little 
or  no  danger  or  an  upward  pressure. 

471.  Retaining  or  Sustaining-  Walls.  These  terms  are 
applied  to  walls  which  sustain  a  lateral  pressure  from  an 
embankment,  or  a  head  of  water. 

472.  Retaining  walls  may  yield  by  sliding  either  along  the 
baie  of  the  foundation  courses,  or  along  one  of  the  horizontal 


240 


CmL  ENGINEERING. 


joints,  or  by  rotation  about  the  exterior  edge  of  some  one  of 
the  horizontal  joints,  or  the  line  of  fracture  may  be  oblique 
to  the  base. 

473.  The  determination  of  the  form  and  dimensions  of  a 
retaining  wall  for  an  embaidvment  of  earth  is  a  problem  of 
considerable  intricacy,  and  the  mathematical  solutions  which 
have  been  given  of  it  have  generally  been  confined  to  particu- 
lar cases,  for  which  approximate  results  alone  have  been  ob- 
tained ;  these,  however,  present  sufticient  accuracy  for  all 
practical  purposes  within  the  limits  to  which  the  solutions  are 
applicable.  Among  the  many  solutions  of  this  problem,  those 
given  by  M.  Poncelet,  of  the  Corps  of  French  Military  En- 
gineei-s,  in  a  Memoir  cm  this  subject,  published  in  the  Me- 
morial de  V  Officier  du  Genie,  No.  10,  present  a  degree  of  re- 
search and  completeness  which  peculiarly  characterize  all 
the  writings  of  this  gentleman,  and  have  given  to  his  produc- 
tions a  claim  to  the  fullest  confidence  of  practical  men. 

The  following  formula,  applicable  to  cases  of  rotation  about 
the  exterior  edge  of  the  lowest  horizontal  joint,  are  taken  fi'om 
the  memoir  above  cited. 

Calling  H,  the  height  BC  (Fig.  65)  of  a  wall  of  uniform 
thickness,  the  face  and  back  being  vertical. 
G.^  —mrnm 


A,  the  mean  height  CG  of  the  embankment,  retained  by  the 
wall,  above  the  top  of  the  wall. 

c,  the  herm  DI,  or  distance  between  the  foot  of  the  embink- 
ment  and  the  outer  edge  of  the  top  of  the  wall, 
the  angle  between  the  line  of  the  natural  slope  BN  of  tlie 
earth  of  the  embankment  and  the  vertical  BG» 

f  =cot.  a,  the  co-efiicient  of  friction  of  the  earth  of  the  em- 
bankment. 

tlie  weight  of  a  cubic  foot  of  the  earth. 


D 


Fig.  65. — Represents  a  section  0  of  a  retaining  waU 
with  the  face  and  back  vertical. 
P,  section  of  the  embankment  above  the  wall. 


RETAINING  WALLS. 


241 


w\  the  weight  of  a  cubic  foot  of  the  masonry  of  the  wall, 
the  base  AB,  or  thickness  of  the  wall  at  bottom. 
Then,  _ 

5=0.74  tan.  ■Ja^^XA+1.126H)  +  0.0488A  —  0.56o  tan.  a(g 

—0.6^)  (A— 0.25). 

w'  H 

The  above  formula  gives  the  value  of  the  base  of  a  wall 
with  vertical  faces,  within  a  near  degree  of  approximation  to 
the  true  result,  only  when  the  values  of  the  quantities  which 
enter  into  it  are  confined  within  certain  limits.  These  limits 
are  as  follows ;  for  A,  between  0  and  H  ;  <?,  between  0  and 
|-H ;  between  0.6  and  1.4,  which  correspond  to  values  of  a 
of  70°  and  35°,  being  in  the  one  case  the  angle  which  the  line 
of  the  natural  slope  of  very  fine  dry  sand  assumes,  and  in  the 
other  of  heavy  clayey  earth ;  and  for  between  w\  and 
Besides  these  limits,  the  formula  also  rests  on  the  assumption 
that  the  moment  of  the  pressure  against  the  wall  is  1.912 
times  the  moment  of  strict  equilibrium  between  it  and  the 
wall.  This  excess  of  stability  given  to  the  wall  supposes  an 
excess  of  resistance  above  the  pressure  against  it  equal  to 
what  obtains  in  the  retaining  walls  of  Yauban,  for  fortifica- 
tions which  have  now  stood  the  test  of  more  than  a  century 
with  security. 

474.  Having  by  the  preceding  formula  calculated  the 
value  of  h  for  a  vertical  wall,  the  base  h'  of  another  wall,  pre- 
senting equal  stability,  but  having  a  batter  on  the  face,  the 
back  being  vertical,  which  is  the  usual  form  of  the  cross  sec- 


Fig.  66— Represents  a  section  0  of  a  retaining  wall  with 

a  sloping  face  AD. 
P,  section  of  the  embankment. 


tion  of  retaining  walls,  can  be  calculated  from  the  following 
notation  and  formula. 

Calling  (Fig.  66)  h'  the  base  of  the  sloping  wall 

16  I 


242 


CIVIL  ENGINEERING. 


^  _       the  batter,  or  ratio  of  the  base  of  the  slope  to  the 

perpendicular,  or  height  of  the  wall. 
Then, 

=  h  +  -f^TlE. 

475.  With  regard  to  sliding  either  on  the  base  of  the 
foundation  courses,  or  on  the  bed  of  any  of  the  horizontal 
joints  of  the  wall,  M.  Poncelet  shows,  in  the  memoir  cited, 
by  a  comparison  of  the  results  obtained  from  calculations 
made  under  the  suppositions  both  of  rotation  and  sliding, 
that  no  danger  need  be  apprehended  from  the  latter,  when 
the  dimensions  are  calculated  to  conform  to  the  former,  so 
long  as  the  limits  of  A  are  taken  between  0  and  411 ;  particu- 
larly if  the  precaution  be  taken  to  allow  the  mortar  of  the 
masonry  to  set  firmly  before  forming  the  embankment  behind 
the  wall. 

476.  Mr.  C.  S.  Constable  read  a  paper  before  the  American 
Society  of  Civil  Engineers  in  New  York,  in  1873,  in  which  he 
showed  by  means  of  a  model  and  experiments  that  the  prism 
which  produces  the  maximum  thrust  or  pressure  was  less  than 
GCD.  The  wall,  when  composed  of  blocks,  will  not  turn 
over  bodily  about  the  outer  edge,  but  there  wjll  be  a  broken 
line  of  fracture  as  shown  by  the  heavy  line  in  (Fig.  67),  the 
general  direction  of  which  corresponds  to  the  natural  slope 


Fig.  67— C  G  is  the  back  of  the  •vrall. 
C  E  represents  the  natural  slope  of  the 
earth.  GCD  the  prism  which  gives 
the  maximum  pressure.  A  B  a  line 
paraUel  to  C  D. 


of  the  earth,  although  the  two  have  not  necessarily  the  same 
direction.  This  being  the  case,  it  is  evident  that  a  portion  of 
the  prism  GCD  will  not  be  active  in  overturning  the  wall,  but 
on  the  other  hand  will  ])revent,  or  tend  to  prevent,  a  portion 
of  the  back  of  the  wall  from  moving  with  the  main  part. 
As  a  result  of  this  investigation  it  is  evident  that  the  for- 
mulas which  are  founded  on  the  supposition  that  the  whole  of 
the  prism  GCD  is  active  in  producing  a  rotation  of  the  wall 


EETAINING  WALLS. 


243 


err  on  the  safe  side,  and  give  an  unnecessarily  large  margin 
for  safety. 

His  experiments  also  showed  that  the  wall  might  start  to 
fall  but  not  fall,  and  that  it  required  considerable  jarring  to 
cause  it  to  fall.  When  the  movement  began  the  face  did  not 
remain  plane  but  became  curved.  This  shows  why  in  prac- 
tice walls  have  assumed  a  curved  face,  and  yet  stand  securely 
for  many  years.  After  a  slight  movement  has  taken  place, 
the  pressure  due  to  the  earth  is  slightly  relieved,  and  the 
whole  mass  takes  up  a  new  position  of  equilibrium,  until 
finally  the  earth  nearly  supports  itself. 

477.  Form  of  Section  of  Retaining  Walls.  The  more 
usual  form  of  cross  section  is  that  in  which  the  back  of  the 
wall  is  built  vertically,  and  the  face  with  a  batter  varying 
between  one  base  to  six  perpendicular,  and  one  base  to 
twenty-four  perpendicular.  The  former  limit  having  been 
adopted,  for  the  reasons  already  assigned,  to  secure  the  joints 
from  the  effects  of  weather ;  and  the  latter  because  a  wall 
having  a  face  more  nearly  vertical  is  liable  in  time  to  yield 
to  the  effects  of  the  pressure,  and  lean  forward. 

478.  The  most  advantageous  form  of  cross  section  for 
economy  of  masonry  is  the  one  (Fig.  68)  termed  a  leaning 


Fig.  68 — Represents  a  section  0  of  a  leaning  retaining 
wall  with  a  sloping  face  AD  and  the  back  BC  coun- 
ter-sloped. 


retaining  wall.  The  counter  slope,  or  reversed  batter  of  the 
back  of  the  wall,  should  not  be  less  than  six  perpendicular  to 
one  base.  In  this  case  strength  requires  that  the  perpendi- 
cular let  fall  from  the  centre  of  gravity  of  the  section  upon 
the  base,  should  fall  so  far  within  the  inner  edge  of  the  base, 
that  the  stone  of  the  bottom  course  of  the  foundation  may 
present  sufficient  surface  to  bear  the  pressure  upon  it. 

479.  Walls  with  a  curved  batter  (Fig.  69)  both  upon  the 
face  and  back,  have  been  used  in  England,  by  some  engineers, 
for  quays.    They  present  no  peculiar  advantages  in  strength 


244 


CIVIL  ENGINEEEING. 


over  walls  with  plane  faces  and  backs,  and  reqnire  particular 
care  in  arranging  the  bond,  and  fitting  the  stones  or  bricks 
of  the  face. 


Fig.  69— Represents  a  section  A  of  a  wall  with  a 
curved  face  and  back,  and  an  elevation  B  of  the 
counterforts. 

C,  water.  ' 

D,  embankment  behind  the  walL 
a,  fender  beams  of  timber. 


480.  Measures  for  increasing  the  Strength  of  Retain- 
ing- Walls,  These  consist  in  the  addition  of  counterforts, 
in  the  use  of  relieving  arches,  and  in  the  modes  of  forming 
the  embankment. 

481.  Counterforts  give  additional  strength  to  a  retaining 
wall  in  several  ways.  By  dividing  the  whole  line  of  the  wall 
into  shorter  lengths  between  each  pair  of  counterforts,  they 
prevent  the  horizontal  courses  of  the  wall  from  yielding  to  the 
pressure  of  the  earth,  and  bulging  outward  between  the  ex- 
tremities of  the  walls  ;  by  receiving  the  pressure  of  the  earth 
on  the  back  of  the  counterfort,  instead  of  on  the  correspond- 
ing portion  of  the  back  of  the  wall,  its  effect  in  producing 
rotation  about  the  exterior  foot  of  the  wall  is  diminished;  the 
Bides  of  the  counterforts  acting  as  abutments  to  the  mass  of 
earth  between  them  may,  in  the  case  of  sand,  or  like  soil, 
cause  the  portion  of  the  wall  between  the  counterforts  to  be 
relieved  from  a  part  of  the  pressure  of  the  earth  behind  it, 
owing  to  the  maimer  in  which  the  particles  of  sand  become 
buttressed  against  each  other  when  confined  laterally,  and 
offer  a  resistance  to  pressure. 

482.  The  horizontal  section  of  counterforts  may  be  either 
rectangular  or  trapezoidal.  When  placed  against  the  back  of 
a  wall,  the  rectangular  form  offers  the  greater  stability  in  the 
case  of  rotation,  and  is  more  economical  in  construction ;  the 
trapezoidal  form  gives  a  broader  and  therefore  a  firmer  con- 


EETAINma  WALLS. 


245 


nection  bet^^een  the  wall  and  counterfort  than  the  rectangular, 
a  point  of  some  consideration  where,  from  the  character  of 
the  materials,  the  strength  of  this  connection  must  mainly  de- 
pend upon  the  strength  of  the  mortar  used  for  the  masonry. 

483.  Counterforts  have  been  chiefly  used  by  military  engi- 
neers for  the  retaining  walls  of  fortifications,  termed  revete- 
ments.  In  regulating  their  form  and  dimensions,  the  practice 
of  Vauban  has  generally  been  followed,  which  is  to  make  the 
horizontal  section  of  the  counterfort  trapezoidal,  making  the 
height  of  the  trapezoid  ^(Fig.  70),  which  corresponds  to  the 


Fig.  70— Represents  a  section  A  and  plan  D  of  a  wall,  and  an 
elevation  B,  and  plan  E  of  a  trapezoidal  comiterfort. 


length  of  the  counterfort,  two-tenths  of  the  height  of  the 
wall  added  to  two  feet,  the  base  of  the  trapezoid  ah  corre- 
sponding to  the  junction  of  the  counterfort  and  back  of  the 
wall,  one-tenth  of  the  height  added  to  two  feet,  and  the  side 
cd  which  corresponds  to  the  back  of  the  counterfort  equal  to 
two-thirds  of  the  base  ah.  The  counterforts  are  placed  from 
15  to  18  feet  from  centre  to  centre  along  the  back  of  the 
wall,  according  to  the  strength  required. 

484.  In  adding  counterforts  to  walls,  the  practice  has  ge- 
nerally been  to  regard  them  only  as  giving  additional  stability 
to  the  wall,  and  not  as  a  means  of  diminishing  its  volume  of 
masonry  of  which  the  addition  of  the  counterforts  ought  to 
admit.  , 

485.  Believing  Arches  are  so  termed  from  their  preventing 
a  portion  of  the  embankment  from  resting  against  the  back 


246 


CIVIL  ENGINEERING. 


of  the  Avail,  and  thus  relieving  it  from  a  part  of  the  pressure. 
They  consist  (Fig.  71)  of  one  or  more  tiers  of  brick  arches 


N 


Fig.  71— Represents  a  section  M  and  an  eleva- 
tion  N  of  a  wall  and  relieving  arches  in  three 
tiers. 

A,  section  of  the  wall. 
.--^^  I  I        c,  c,  c,  sections  of  the  arches  through  their 
' rC-—  crowns. 

^  ^     d,  rf,  interior  elevations  of  counterforts  serving 

as  piers  of  the  arches. 

interior  end  elevations  of  arches. 


built  upon  counterforts,  vrhich  act  as  the  piers  of  the  arches. 

In  arranging  a  combination  of  relieving  arches  and  their 
piei*s,  the  latter,  like  ordinary  counterforts,  are  placed  about 
18  feet  apart  between  their  centre  lines  ;  their  length  should 
be  so  regulated  that  the  earth  behind  them  resting  on  the 
arches,  and  falling  under  them  v^rith  the  natural  slope,  shall 
not  reach  the  vrall  between  the  arch  and  the  foot  of  the  back 
of  the  wall  below  the  arch.  The  thickness  of  the  arches,  as 
well  as  that  of  the  counterforts,  will  depend  upon  the  weight 
which  the  arches  sustain.  The  dimensions  of  the  wall  will 
be  regulated  by  the  decreased  pressure  against  it  caused  by 
the  action  of  the  arches,  and  the  point  at  which  this  pressm-e 
acts. 

486.  Whenever  it  becomes  necessary  to  form  the  embank- 
ment before  the  mortar  of  the  retaining  wall  has  had  time  to 
set  firmly,  the  portion  of  the  embankment  next  to  the  wall 
may  be  of  a  compact  binding  earth  placed  in  layers  inclining 
downward  from  the  back  of  the  wall,  and  well  rammed  ;  or 
of  a  stiff  mortar  made  either  of  clay,  or  sand,  with  about  -g-Vth 
in  bulk  of  lime.  Instead  of  brino^ino^  the  embankment  di- 
rectly  against  the  back  of  the  wall,  dry  stone,  or  fascines  may 
be  laid  in  to  a  suitable  depth  back  from  the  w^all  for  the  same 
purpose.  The  precautions  however^  of  allowing  the  mortar 
to  set  firmly  hef ore  forming  the  emhankmeiit,  should  never  be 
omitted  except  in  cases  of  extreme  urgency ^  and  then  the 
bond  of  the  masonry  should  be  arranged  with  peculiar  care, 
to  prevent  disjunction  along  any  of  the  horizontal  joints. 

487.  Walls  built  to  sustain  a  pressure  of  water  should  be 
regulated  in  form  and  dimensions  like  the  retaining  walls  of 
embankments.  The  buoyant  effort  of  the  water  must  be 
taken  into  account  in  determining  the  dimensions  of  the 
wall,  whenever  the  masonry  is  so  placed  as  to  be  partially 
immersed  in  the  water. 


ARCHES. 


247 


488.  Heavy  walls,  and  even  those  of  ordinary  dimensions, 
when  exposed  to  moisture,  should,  be  laid  in  hydraulic  mortar. 
Grout  has  been  tried  in  laying  heavy  rubble  walls,  but  with 
decided  want  of  success,  the  successive  drenchings  of  the 
stone  causing  the  sand,  to  separate  from  the  lime,  leaving 
when  dry  a  weak  porous  mortar.  When  the  stone  is  laid  in 
full  mortar,  grout  may  be  used  with  advantage  over  each 
course,  to  fill  any  voids  left  in  the  mass. 

489.  Beton  has  frequently  been  used  as  a  filling  between 
the  back  and  facing  of  water-tight  walls ;  it  presents  no  ad- 
vantage over  walls  of  cut,  or  rubble  stone  laid  in  hydraulic 
mortar,  and  causes  unequal  settling  in  the  parts,  unless  great 
care  is  taken  in  the  construction. 

490.  When  a  weight,  arising  from  a  mass  of  masonry  or 
earth,  rests  upon  two  or  more  isolated  supports,  that  portion 
of  it  which  is  distributed  over  the  space,  or  hearing  between 
any  two  of  the  supports,  may  be  borne  by  a  block  of  stone, 
termed  a  lintel^  laid  horizontally  upon  the  supports,  by  a 
combination  of  blocks  termed  2, j^late-bande^  so  arranged  as  to 
resist,  without  disjunction,  the  pressure  upon  them ;  or  by  an 
arch. 

491.  Lintel.  Owing  to  the  slight  resistance  of  stone  to  a 
cross  strain,  and  to  shocks,  lintels  of  ordinary  dimensions 
cannot  be  used  alone  with  safety,  for  bearings  over  five  or 
six  feet.  For  wider  bearings,  a  slight  brick  arch  is  thrown 
across  the  bearing  above  the  lintel,  and  thus  relieves  it  from 
the  pressure  of  the  parts  above. 

492.  Plate-bande.  The  plate-bande  is  a  combination  of 
blocks  cut  in  the  form  of  truncated  wedges.  From  the  form 
of  the  blocks,  the  pressure  thrown  upon  them  causes  a  lateral 
pressure  which  must  be  sustained  either  by  the  supports,  or 
by  some  other  arrangement  (Fig.  72). 


Fig.  72— Represents  a  cross 
section  of  a  plate-bande, 
sliowing  the  manner  ia 
which  the  voussoira  A,  A 
and  B  are  cut  and  con- 
nected by  metal  cramps. 
a&,  tie  of  wrought  iron  for 
the  plate-bande  fastened 
to  the  bolts  cd,  let  into 
the  piCTS  of  the  plate- 
bande. 

The  plate-bande  should  be  used  only  for  narrow  bearings, 
as  the  upper  edges  of  the  blocks  at  the  acute  angles  are  liable 
to  spUnter  from  the  pressure.  If  the  bearing  exceeds  10 
feet,  the  plate-bande  should  be  relieved  from  the  pressure 


248 


CIVIL  ENGINEERING. 


by  a  brick  arch  above  it.  Additional  means  of  strengthening 
the  plate-baiide  are  sometimes  used  by  forming  a  broken  joint 
between  the  blocks,  or  by  a  projection  made  on  the  face  of 
one  block  to  fit  into  a  corresponding  indent  in  the  adjacent 
one,  or  by  connecting  the  blocks  with  iron  bolts. 

When,  from  any  cause,  the  supports  cannot  be  made  suffi- 
ciently strong  to  resist  the  lateral  pressure  of  the  phite-baude, 
the  extreme  blocks  must  be  united  by  an  iron  bar,  termed  a 
tie,  suitably  arranged  to  keep  the  blocks  from  yielding. 

493.  Arches.  The  arch  is  a  combination  of  wedge-shaped 
blocks,  termed  arch  stones,  or  voussoirs,  truncated  towards 
the  angle  of  the  wedges  by  a  curved  surface  which  is  usually 
normal  to  the  surfaces  of  the  joints  between  the  blocks. 
This  inferior  surface  of  the  arch  is  termed  the  soj/it.  The 
upper,  or  outer  surface  of  the  arch  is  termed  the  back 
(Fig.  73). 


Fig.  73— Eepresents  an  elevation  M  of  the  head  of  a  right  cylindrical  arch, 
and  a  section  N  through  the  crovra  of  the  arch  A,  with  an  elevation  B  of 
the  soffit  and  the  face  C  of  the  abutment. 

oft,  span  of  the  arch, 

dc,  rise, 

ac6,  curve  of  the  intrados. 
e,  e,  voussoirs  forming  ring  courses  of  heads. 
/,  key  stone. 

gr,  cushion  stone  of  abutment. 
»nre,  crown  of  the  arch. 
op,  springing  line. 


494.  The  extreme  blocks  of  the  arch  rest  against  lateral 
supports,  termed  ahutments,  which  sustain  both  the  vertical 
pressure  arising  from  the  weight  of  the  arch  stones,  and  the 
weight  of  whatever  lies  upon  them  ;  also  the  lateral  pressure 
caused  by  the  action  of  the  arch. 

495.  In  a  range,  or  series  of  arches  placed  side  by  side, 
the  extreme  supports  are  termed  the  abutments,  the  interme- 
diate supports  which  sustain  the  intermediate  arches  and  the 
halves  of  the  two  extreme  ones  are  termed  piers.  When  the 
size  of  the  arches  is  the  same,  and  their  springing  lines  are 


AEOHES. 


in  the  same  horizontal  plane,  the  piers  receive  no  other  pres- 
sure but  that  arising  from  the  weight  of  the  arches. 

496.  Arches  are  classified,  from  the  form  of  the  soffit,  into 
cylindrical^  conical^  conoidal^  warjped^  annular^  groined^  clois- 
tered, and  domes.  They  are  also  termed  right,  ohlique,  or 
askew,  and  rampant,  from  their  direction  with  respect  to  a 
vertical,  or  horizontal  plane. 

497.  Cylindrical,  groined  and  cloistered  arches  are  formed  by 
the  intersections  of  two  or  more  cylindrical  arches.  The 
span  of  the  arches  may  be  different,  but  the  rise  is  the  same 
in  each.  The  axes  of  the  cylinders  will  be  in  the  same  plane, 
and  they  may  intersect  under  any  angle. 

The  groined  arch  (Fig.  74)  is  formed  by  removing  those 


Fig.  74 — Represents  the  plan  of  the  soflSt 
and  the  right  sections  M  and  N  of  the  cyl- 
inders forming  a  groined  arch. 

Gff,  pillars  supporting  the  arch. 

&c,  groins  of  the  soffit. 

OTO,  mn,  edges  of  coursing  joints. 

A,  key- stone  of  the  two  arches  formed  of 
one  block. 

B,  B,  groin  stones  of  one  block  below  the 
key-stone  forming  a  part  of  each  arch. 


■portions  of  each  cylinder  which  lie  under  the  other  and  be- 
tween their  common  curves  of  intersection  ;  thus  forming  a 
projecting,  or  salient  edge  on  the  soffit  along  these  curves. 

The  cloistered  arch  (Fig.  75)  is  formed  by  removing  those 
portions  of  each  cylinder  which  are  above  the  other  and  ex- 
terior to  their  common  intersection,  forming  thus  re-entering 
angles  along  the  same  lines. 

498.  The  planes  of  the  joints  in  both  of  these  arches  are 
placed  in  the  same  manner  as  in  the  simple  cylindrical  arch. 
The  inner  edges  of  the  corresponding  course  of  vonssoirs  in 
each  arch  are  placed  in  the  same  plane  parallel  to  that  of  the 
axes  of  the  cylinders.  The  portions  of  the  soffit  in  each  cyl- 
inder, corresponding  to  each  course  of  voussoirs,  which  form 
either  the  groin  in  the  one  case,  or  the  re-entering  angle  in 
the  other,  are  cut  from  a  single  stone,  to  present  no  joint 
along  the  common  intersection  of  the  arches,  and  to  give 
them  a  firmer  bond. 


250 


CIVIL  ENGINEEEING. 


499.  When  the  spans  at  the  two  ends  of  an  arch  are  un- 
equal,  but  the  rise  is  the  same,  then  the  soffit  of  the  arch  is 
made  of  a  conoidal  surface.  The  curves  of  right  section  at 
the  two  ends  may  be  of  any  figure,  but  are  usually  taken 
from  some  variety  of  the  elliptical,  or  oval  curves.  The 
soffit  is  formed  by  moving  a  line  upon  the  two  curves,  and 
parallel  to  the  plane  containing  their  spans 

The  conoidal  arch  belongs  to  the  class  with  warped  soffits. 
A  variety  of  warped  surfaces  may  be  used  for  soffits  accord- 
ing to  circumstances ;  the  joints  and  the  bond  depending  on 
the  generation  of  the  surface. 

500.  In  arranging  the  joints  in  conoidal  arches,  the  heading 
joints  are  contained  in  planes  perpendicular  to  the  axis  of 
the  arch.  The  coursing  joints  are  also  foi-med  of  plane  sur- 
faces, so  arranged  that  the  portion  of  the  joint  corresponding 
to  each  block  is  formed  by  a  plane  normal  to  the  conoid  at 
the  middle  point  of  the  lower  edge  of  the  block.  In  this  way 
the  joints  of  the  string  course  w^ill  not  be  formed  of  contin-' 
uous  surfaces.  To  make  them  so,  it  would  be  necessary  to 
give  them  the  form  of  warped  surfaces,  which  present  more 
difficulty  in  their  mechanical  execution,  and  not  sufficient  ad- 
vantages over  the  method  just  explained  to  compensate  for 
Laving  them  continuous. 

501.  The  annular  arch  is  formed  by  revolving  the  plane  o£ 
a  semi-circle,  or  semi-oval,  or  other  curve,  about  a  line  di-awn 


ARCHES. 


2511 


without  the  figure  and  parallel  to  the  rise  of  the  arch  (Fig. 
76)  One  series  of  -  joints  in  this  arch  will  be  formed  by 
conical  surfaces  passing  through  the  inner  edges  of  the 
stones  which  correspond  to  the  string  courses  ;  and  the  otiier 
series  will  be  planes  passed  through  the  axis  about  which  the 
semi-circle  is  revoh  ed.  This  last  series  should  break  ]omts 
with  each  other. 


502.  The  soffit  of  a  dome  is  usually  formed  by  revolving 
the  quadrant  of  one  of  the  usual  curves  of  cylindrical  arches 
around  the  rise  of  the  curve ;  or  else  by  revolving  the  semi- 
curve  about  the  line  of  the  span,  and  taking  the  half  of  the 
surface  thus  generated  for  the  sofiit  of  the  dome.  In  the 
first  of  these  cases  the  horizontal  section  of  the  dome  at  the 
springing  line  will  be  a  circle ;  in  the  second  the  entire  curve 
of  the  semi-curve  by  which  the  sofiit  is  generated.  The  plan 
of  domes  may  also  be  of  regular  polygonal  figures,  in  which 
case  the  sofiit  will  be  a  polygonal-cloistered  arch  formed  of 
equal  sections  of  cylinders  (Fig.  77).  The  joints  and  the 
bond  are  determined  in  the  same  manner  as  in  other  arches,  j 

503.  The  voussoirs  which  form  the  ring  course  of  the 
heads,  in  ordinary  cylindrical  arches,  are  usually  terminated 
by  plane  surfaces  at  top  and  on  the  sides,  for  the  purpose  of 
connecting  them  with  the  horizontal  courses  of  the  head  which 


252 


OIVJLL  ENGHTEEEING. 


lie  above  and  on  each  side  of  the  arch  (Figs.  78  and  79). 
This  connection  may  be  arranged  in  a  variety  of  ways.  The  two 
points  to  be  kept  in  view  are,  to  form  a  good  bond  between 
the  Youssoirs  and  horizontal  courses,  and  to  give  a  pleasing 


Fig.  78 — Represents  a  manner  of  connecting  the  voussoirs  and 

horizontal  courses  in  an  oval  arch, 
o,  o,  are  examples  of  voussoirs  with  elbow  joints. 


Fig.  79 — Represents  a  mode  of  arranging  the  voussoirs  and 

horizontal  courses  in  flat  segment  arches. 


architectural  effect  by  the  arrangement.  This  connection 
should  always  give  a  symmetrical  appearance  to  the  halves  of 
the  structure  on  each  side  of  the  crown.  To  effect  these 
several  objects  it  may  be  necessary,  in  cases  of  oval  arches,  to 
make  the  breadth  of  the  voussoirs  unequal,  diminishing 
usually  those  near  the  springing  lines. 

504.  In  small  arches  the  voussoirs  near  the  springing  line 
are  so  cut  as  to  form  a  part  also  of  the  horizontal  course  (see 
Fig.  78),  forming  what  is  termed  an  elbow  joint.  This  plan 
is  objectionable,  both  because  there  is  a  waste  of  material  in 
forming  a  joint  of  this  kind,  and  the  stone  is  liable  to  crack 
when  the  arch  settles. 

505.  The  forms  and  dimensions  of  the  voussoirs  should  be 
determined  both  by  geometrical  drawings  and  numerical 


ARCHES. 


253 


calculation,  whenever  the  arch  is  important,  or  presents  any 
complication  of  form.  The  drawings  should,  in  the  first  place, 
be  made  to  a  scale  sufficiently  large  to  determine  the  parts 
with  accuracy,  and  from  these,  pattern  drawings  giving  the 
parts  in  their  true  size  may  be  made  for  the  use  of  the  mason. 
To  make  the  pattern  drawings,  the  side  of  a  vertical  wall,  or 
a  firm  horizontal  area  may  be  prepared,  with  a  thin  coating 
of  mortar,  to  receive  a  thin  smooth  coat  of  plaster  of  Paris. 
The  drawing  may  be  made  on  this  surface  in  the  usual  man- 
ner, by  describing  the  curve  either  by  points  from  its  calcu- 
lated abscissas  and  ordinates,  or,  where  it  is  formed  of  circular 
arcs,  by  using  the  ordinary  instrument  for  describing  such 
arcs  when  the  centres  fall  within  the  limits  of  the  prepared 
surface.  In  ovals  the  positions  of  the  extreme  radii  should  be 
accurately  drawm  either  from  calculation,  or  construction. 
To  construct  the  intermediate  normals,  whenever  the  centres 
of  the  arcs  do  not  fall  on  the  surface,  an  arc  with  a  chord  of 
about  one  foot  may  be  set  oif  on  each  side  of  the  point 
through  which  the  normal  is  to  be  drawn,  and  the  chord  of 
the  whole  arc,  thus  set  ofi^,  be  bisected  by  a  perpendicular. 
This  construction  w^ill  generally  give  a  sufficiently  accurate 
practical  result  for  elliptical  and  other  curves  of  a  large  size. 

506.  The  masonry  of  arches  may  be  either  of  dressed  stone, 
rubble,  or  brick. 

In  wide  spans,  particularly  for  oval  and  other  flat  arches, 
cut  stone  should  alone  be  used.  The  joints  should  be  dressed 
with  extreme  accuracy.  As  the  voussoirs  have  to  be  sup- 
ported by  a  framing  of  timber,  termed  a  centre^  until  the 
arch  is  completed,  and  as  this  structure  is  liable  to  yield,  both 
from  the  elasticity  of  the  materials  and  the  number  of  joints 
in  the  frame,  an  allowance  for  the  settling  in  the  arch,  arising 
from  these  causes,  is  sometimes  made,  in  cutting  the  joints  of 
the  voussoirs  that  is,  not  according  to  the  true  position 

of  the  normal,  but  from  the  supposed  position  the  joints  will 
take  when  the  arch  has  settled  thoroughly.  The  object  of 
this  is  to  bring  the  surfaces  of  the  joints  into  perfect  contact 
when  'the  arch  has  assumed  its  permanent  state  of  equilibrium, 
and  thus  prevent  the  voussoirs  from  breaking  by  unequal 
pressures  on  their  coursing  joints.  This  is  a  problem  of  con- 
siderable difficulty,  and  it  will  generally  be  better  to  cut  the 
joints  true,  and  guard  against  settling  and  its  effects  by  giving 
great  stiffness  to  the  centres,  and  by  placing  between  the 
joints  of  those  voussoirs,  where  the  principal  movement  takes 
place  in  arches,  sheets  of  lead  suitably  hammered  to  fit  the 
joint  and  yield  to  any  pressure. 


254 


Cn^IL  ENGINEERING. 


607.  The  manner  of  laying  the  voussoirs  demands  peculiar 
care,  particularly  in  those  which  form  the  heads  of  the  arch. 
The  positions  of  the  inner  ed^^es  of  the  voussoirs  are  deter- 
mined by  fixed  lines,  marked  on  the  abutments,  or  some 
other  immovable  object,  and  the  calculated  distances  of  the 
edges  from  these  lines.  These  distances  can  be  readily  set 
off  by  means  of  tlie  level  and  plumb-line.  The  angle  of  each 
joint  can  be  fixed  by  a  quadrant  of  a  circle,  connected  with  a 
plumb-line,  on  which  the  position  of  each  joint  is  marked. 

508.  Brick  may  be  used  alone,  or  in  combination  with  cut 
stone,  for  arches  of  considerable  size.  When  the  thickness  of 
a  brick  arch  exceeds  a  brick  and  a  half,  the  bond  from  the 
softit  outward  presents  some  diflSculties.  If  the  bricks  are 
laid  in  concentric  layers,  or  shells^  a  continuous  joint  w^ill  be 
formed  parallel  to  the  surface  of  the  sofiit,  which  will  proba- 
bly yield  when  the  arch  settles,  causing  the  shells  to  separate 
(Fig.  80).    If  the  bricks  are  laid  like  ordinary  string  courses, 


N 


Fig.  80— Represents  an  end 
view,  M,  of  a  brick  arch 
built  with  blocks,  C,  and 
shells,  A  and  B. 

N,  represents  the  manner 
of  arranging  the  courses 
of  brick  forming  the 
crown  of  the  arch. 


forming  continuous  joints  from  the  sofiRt  outward,  these  joints, 
from  the  form  of  the  bricks,  will  be  very  open  at  the  back, 
and,  from  the  yielding  of  the  mortar,  the  arch  will  be  liable 
to  injury  in  settling  from  this  cause.  To  obviate  both  of  these 
defects,  the  arch  may  be  built  partly  by  the  first  plan  and 
partly  b}^  the  second,  or  as  it  is  termed  in  shells  and  blocks. 
The  crown,  or  key  of  the  arch  should  be  laid  in  a  block,  in- 
creasing the  breadth  of  the  block  by  two  bi-icks  for  each 
course  from  the  softit  outward.  These  bricks  should  be  laid 
in  hydraulic  cement,  and  be  well  wedged  with  pieces  of  thin 
hard  slate  between  the  joints. 

609.  When  a  combination  of  brick  and  cut  stone  is  used,  the 


AECHES. 


255 


ring  courses  of  the  heads,  with  some  intermediate  ring  courses, 
the  bottom  string  courses,  the  keystone  course,  and  a  few  in- 
termediate string  courses,  are  made  of  cut  stone  (Fig.  81),  the 


Fig.  81  —  Kepre- 
eents  a  cross  sec- 
tion of  a  stone 
segment  arch, 
capped  with 
brick  and  beton, 

A,  stone  voiissoirs. 

B  and  D,  brick  and 
beton  capping. 

C,  abutment. 

£,  cushion  stone. 


intermediate  spaces  being  filled  in  with  brick.    The  brick 

portions  of  the  soffit  may,  if  necessary,  be  thrown  within  the 
stone  portions,  forming  plain  caissons. 

510.  The  centres  of  large  arches  should  not  be  struck  until 
the  whole  of  the  mortar  has  set  firmly.  After  the  centres 
are  struck,  the  arch  is  allowed  to  assume  its  permanent  state 
of  equilibrium,  before  any  of  the  superstructure  is  laid. 

511.  When  the  heads  of  the  arch  form  a  part  of  an  exterior 
surface,  as  the  faces  of  a  wall,  or  the  outer  portions  of  a 
bridge,  the  voussoirs  of  the  head  ring  courses  are  connected 
with  the  horizontal  courses,  as  has  been  explained  ;  the  top 
surface  of  the  voussoirs  of  the  intermediate  ring  courses  are 
usually  left  in  a  roughly  dressed  state  to  receive  the  courses 
of  masonry  termed  the  capping  (see  Fig.  81),  which  rests 
upon  the  arch  between  the  walls  of  the  head.  Before  laying 
the  capping,  the  joints  of  the  voussoirs  on  the  back  of  the 
arch  should  be  carefully  examined,  and,  wherever  they  are 
found  to  be  open  from  the  settling  of  the  arch,  they  should 
be  filled  up  with  soft-tempered  mortar,  and  by  driving  in 
pieces  of  hard  slate.  The  capping  may  be  variously  formed 
of  rubble,  brick,  or  beton.  Where  the  arches  are  exposed  to 
the  filtration  of  rain  water,  as  in  those  used  for  bridges  and 
the  casemates  of  fortifications,  the  capping  should  be  of  beton 
laid  in  layers,  and  well  rammed,  with  the  usual  precautions 
for  obtaining  a  solid  homogeneous  mass. 

512.  The  difficulty  of  forming  water-tight  cappings  of 
masonry  has  led  engineers,  within  a  few  years  back,  to  try  a 
coating  of  asphalte  upon  the  surface  of  beton.    The  surface 


256 


CIVIL  ENGINEERING. 


of  the  beton  capping  is  made  tiniform  and  smooth  by  the 
trowel,  or  float,  and  the  mass  is  allowed  to  become  thoroughly 
dry  before  the  asphalte  is  laid.  Asphalte  is  usually  laid  on  in 
two  layers.  Before  applying  the  first,  the  surface  of  the 
beton  should  be  thorouglily  cleansed  of  dust,  and  receive  a 
coating  of  mineral  tar  applied  hot  with  a  swab.  This  appli- 
cation of  hot  mineral  tar  is  said  to  prevent  the  formation  of 
air  bubbles  in  the  layers  of  asphalte  which,  when  present, 
permit  the  water  to  percolate  through  the  masonry.  The  first 
layer  of  asphalte  is  laid  on  in  squares,  or  thin  blocks,  care 
being  taken  to  form  a  perfect  union  between  the  edges  of 
the  squares  by  pouring  the  hot  liquid  along  them  in  forming 
each  new  one.  The  surface  of  the  first  layer  is  made  uni- 
form, and  rubbed  until  it  becomes  smooth  and  hard  ^vith  an 
ordinary  wooden  float.  In  laying  the  second  layer,  the  same 
precautions  are  taken  as  for  the  first,  the  squares  breaking 
joints  with  those  of  the  first.  Fine  sand  is  strewed  over  the 
surface  of  the  top  layer,  and  pressed  into  the  asphalte  before 
it  becomes  hard. 

Coverings  of  asphalte  have  been  used  both  in  Europe  and 
in  our  military  structures  for  some  years  back  with  decided 
success.  There  have  been  failures,  in  some  instances,  arising 
in  all  probability  either  from  using  a  bad  material,  or  from 
some  fault  of  w^orkmanship. 

513.  In  a  range  of  arches,  like  those  of  bridges,  or  case- 
mates, the  capping  of  each  arch  is  shaped  w^ith  two  inclined 
surfaces,  like  a  common  roof.  The  bottoms  of  these  surfaces, 
by  their  junction,  form  gutters  where  the  water  collects,  and 
from  which  it  is  conveyed  off  in  conduits,  formed  either  of 
iron  pipes,  or  of  vertical  openings  made  through  the  masonry 
of  the  piers  which  communicate  with  horizontal  covered 
drains.  A  small  arch  of  sufticient  width  to  admit  a  man  to 
examine  its  interior,  or  a  square  culvert,  is  formed  over  the 
gutter.  When  the  spaces  between  the  head  walls  above  the 
capping  is  filled  in  with  earth,  a  series  of  drains  running 
from  the  top,  or  of  the  capping,  and  leading  into  the 
main  gutter  drain,  should  be  formed  of  brick.  They  may  be 
best  made  by  using  dry  brick  laid  flat,  and  with  intervals  left 
for  the  drains,  these  being  covered  by  other  courses  of  dry 
brick  with  the  joints  in  some  degree  open.  The  earth  is  filled 
in  upon  the  upper  course  of  bricks,  which  should  be  so  laid 
as  to  form  a  uniform  surface. 

514.  From  observations  taken  on  the  manner  in  which 
large  cylindrical  arches  settle,  and  experiments  made  on  a 
small  scale,  it  appears  that  in  all  cases  of  arches  where  the 


ARCHES. 


257 


rise  is  eqnal  to  or  less  than  the  half  span  they  yield  (Fig.  82) 
by  the  crown  of  the  arch  falling  inward,  and  thrusting  out- 
ward the  lower  portions,  presenting  five  joints  of  rupture, 
one  at  the  keystone,  one  on  each  side  of  it  which  limit  the 
portions  that  fall  inward,  and  one  on  each  side  near  the 
Bpringing  lines  which  limit  the  parts  thrust  outward.  In 


Fig.  82 — Represents  the  manner  In  which  flat  arches 

yield  by  rupture, 
o,  joint  of  rupture  at  the  keystone, 
m,  m,  joints  of  rupture  below  the  keystone. 
^   n,  n,  joints  of  rupture  at  springing  lines. 


pointed  arches,  or  those  in  which  the  rise  is  greater  than  the 
half  span,  the  tendency  to  yielding  is,  in  some  cases,  differ- 
ent; here  the  lower  parts  may  fall  inward  (Fig.  83),  and 
thrust  upward  and  outward  the  parts  near  the  crown. 


Fig.  83— Represents  the  manner  in  which  pointed 

arches  may  yield. 
The  letters  refer  to  same  points  as  in  Fig.  82. 


The  angle  which  a  line  drawn  from  the  centre  of  the  arch 
to  the  joint  of  rupture  makes  with  a  vertical  line  is  called 
the  angle  of  rujpUire.  This  term  is  also  used  when  the  arch 
is  stable,  or  when  there  is  no  joint  of  rupture,  in  which  case 
it  refers  to  that  point  about  which  there  is  the  greatest  ten- 
dency to  rotate.  It  may  also  be  defined  as  including  that 
portion  of  the  arcli  near  the  crown  which  w^ill  cause  the 
greatest  thrust  or  horizontal  pressure  at  the  crowm.  This 
thrust  tends  to  crush  the  voussoirs  at  the  crown,  and  also  to 
overturn  the  abutments  about  some  outer  joint.  The  thrust 
is  rarely  sufiicient  to  crush  ordinary  stone.  The  most  com- 
mon mode  of  failure  is  by  rupturing,  or  turning  about  a  joint. 
In  very  thick  arches  rupture  may  take  place  from  slvpping 
on  the  joints. 

515.  The  joints  of  rupture  below  the  keystone  vary  in 
arches  of  different  thicknesses  and  forms,  and  in  the  same 
arch  with  the  weight  it  sustains. 

516.  The  problem  for  finding  the  joints  of  rupture  by  cal- 
culation, and  the  consequent  thickness  of  the  abutments  ne- 
cessary to  preserve  the  arch  from  yielding,  has  been  solved 

17 


258 


CIVIL  ENGINEEEING. 


by  a  immber  of  writers  on  the  theory  of  the  equilibrium  of 
arches,  and  tables  for  effecting  the  necessary  numerical  cal- 
culations have  been  drawn  up  from  their  results  to  abridge 
the  labor  in  each  case. 

517.  In  cloistered  arches  the  abutments  may  be  less  than 
in  an  ordinary  cylindrical  arch  of  the  same  length ;  and  in 
groined  arches,  in  calculating  the  resistance  offered  by  the 
abutments,  the  counter  resistance  offered  by  the  weight  of 
one  portion  in  resisting  the  thrust  of  the  other,  must  be  taken 
into  consideration. 

518.  When  abutments,  as  in  the  case  of  edifices,  require  to 
be  of  considerable  height,  and  therefore  would  demand  ex- 
traordinary thickness,  if  used  alone  to  sustain  the  thrust  of 
the  arch,  they  may  be  strengthened  by  the  addition  to  their 
weight  made  in  carrying  them  up  above  the  imposts  like  the 
'battlements  and  pinnacles  in  Gothic  architecture  ;  by  adding 
to  them  ordinary,  full,  or  arched  buttresses,  termed  flying 
buttresses  /  or  by  using  ties  of  iron  connecting  the  voussoirs 
near  the  joints  of  rupture  below  the  keystone.  Tie-rods  are 
evidently  the  safest  expedient.  The  employment  of  these 
different  expedients,  their  forms  and  dimensions,  will  depend 
on  the  character  of  the  structure  and  the  kind  of  arch.  The 
iron  tie,  for  example,  cannot  be  hidden  from  view  except  in 
the  plate-bande,  or  in  very  flat  segment  arches,  and  wherever 
its  appearance  would  be  unsightly  some  other  expedient  must 
be  tried. 

Circular  rings  of  iron  have  been  used  to  strengthen  the 
abutments  of  domes,  by  confining  the  lower  courses  of  the 
dome  and  relieving  the  abutment  from  the  thrust. 

519.  In  a  range  of  arches  of  unequal  size,  the  piers  will 
have  to  sustain  a  lateral  pressure  occasioned  by  the  unequal 
horizontal  thrust  of  the  arches.  In  arranging  the  form  and 
dimensions  of  the  piers  this  inequality  of  thrust  must  be 
estimated  for,  taking  also  into  consideration  the  position  of 
the  imposts  of  the  unequal  arches. 

520.  Precautions  against  Settling.  One  of  the  most  dif- 
ficult and  important  problems  in  the  construction  of  masonry, 
is  that  of  preventing  unequal  settling  in  parts  which  require 
to  be  connected  but  sustain  unequal  weights,  and  the  conse- 
quent ruptures  in  the  masses  arising  from  this  cause.  To 
obviate  this  difficulty  requires  on  the  part  of  the  engineer  no 
small  degree  of  practical  tact.  Several  precautions  must  be 
taken  to  diminish  as  far  as  practicable  the  danger  from  un- 
equal settling.  Walls  sustaining  heavy  vertical  pressures 
should  be  built  up  uniformly,  and  with  great  attention  to  the 


ARCHES. 


259 


bond  and  correct  fitting  of  the  courses.  The  materials  should 
be  uniform  in  quality  and  size  ;  hydraulic  mortar  should 
alone  be  used  ;  and  the  permanent  weight  not  be  laid  on  the 
wall  until  the  season  after  the  masonry  is  laid.  As  a  farther 
precaution,  when  practicable,  a  trial  weight  may  be  laid  upon 
the  wall  before  loading  it  with  the  permanent  one. 

Where  the  heads  of  arches  are  built  into  a  wall,  particularly 
if  they  are  designed  to  bear  a  heavy  permanent  weight,  as 
an  embankment  of  earth,  the  wall  should  not  be  carried  up 
higher  than  the  imposts  of  the  arches  until  the  settling  of  the 
latter  has  reached  its  final  term ;  and  as  there  will  be  danger 
of  disjunction  between  the  piers  of  the  arches  and  the  wall  at 
the  head,  from  the  same  cause,  these  should  be  carried  up  in- 
dependently, but  so  arranged  that  their  after-union  may  be 
conveniently  effected.  It  would  moreover  be  always  well  to 
suspend  the  building  of  the  arches  until  the  season  follow- 
ing that  in  which  the  piers  are  finished,  and  not  to  place  the 
permanent  weight  upon  the  arches  until  the  season  following 
their  completion. 

521.  Pointing.  The  mortar  in  the  joints  near  the  surfaces 
of  walls  exposed  to  the  w^eather  should  be  of  the  best  hydrau- 
lic lime,  or  cement,  and  as  this  part  of  the  joint  always 
requires  to  be  carefully  attended  to,  it  is  usually  filled,  or  as 
it  is  termed  pointed^  some  time  after  the  Other  work  is  finish- 
ed. The  period  at  which  pointing  should  be  done  is  a  dis- 
puted subject  among  builders,  some  preferring  to  point  while 
the  mortar  in  the  joint  is  still  fresh,  or  green^  and  others  not 
until  it  has  become  hard.  The  latter  is  the  more  usual  and 
better  plan.  The  mortar  for  pointing  should  be  poor,  that  is, 
have  rather  an  excess  of  sand  ;  the  sand  should  be  of  a  fine 
uniform  grain,  and  but  little  water  be  used  in  tempering  the 
mortar.  Before  applying  the  pointing,  the  joint  should  be 
well  cleansed  by  scraping  and  brushing  out  the  loose  matter, 
and  then  be  well  moistened.  The  mortar  is  applied  with  a 
suitable  tool  for  pressing  it  into  the  joint,  and  its  surface  is 
rubbed  smooth  with  an  iron  tool.  The  practice  among  our 
military  engineers  is  to  use  the  ordinary  tools  for  calking  in 
applying  pointing ;  to  calk  the  joint  with  the  mortar  in  the 
usual  way,  and  to  rub  the  surface  of  the  pointing  until  it  be- 
comes hard.  To  obtain  pointing  that  will  withstand  the 
vicissitudes  of  our  climate  is  not  the  least  of  the  difficulties 
of  the  builder's  art.  The  contraction  and  expansion  of  the 
stone  either  causes  the  pointing  to  crack,  or  else  to  separate 
from  the  stone,  and  the  surface  water  penetrating  into  the 
cracks  thus  made,  when  acted  upon  by  frost,  throws  out  the 


260 


CIVIL  ENGINEERING. 


pointiiii^.  Some  have  tried  to  meet  this  difficulty  by  giving 
the  suriace  of  the  pointing  such  a  shape,  and  so  arranging  it 
with  respect  to  the  surfaces  of  the  stones  forming  the  joint, 
that  the  water  shall  trickle  over  the  pointing  without  enter- 
ing the  crack,  wliich  is  usually  between  the  bed  of  the  stone 
and  the  pointing. 

522.  The  term  flash  pointing  is  sometimes  applied  to  a 
coating  of  hydraulic  mortar  laid  over  the  face  or  back  of  a 
wall,  to  preserve  either  the  mortar  joints,  or  the  stone  itself, 
from  the  action  of  moisture,  or  the  effects  of  the  atmosphere. 
Mortar  for  flash  pointing  should  also  be  made  poor,  and  when 
it  is  used  as  a  stucco  to  protect  masonry  from  atmospheric 
action,  it  should  be  made  of  coarse  sand,  and  be  applied  in  a 
single  uniform  coat  over  the  surface,  which  should  be  prepared 
to  receive  the  stucco  by  having  the  joints  thoroughly  cleansed 
from  dust  and  loose  mortar,  and  being  well  moistened. 

No  pointing  of  mortar  has  been  found  to  withstand  the 
effects  of  weather  in  our  climate  on  a  long  line  of  coping. 
Within  a  few  years  a  pointing  of  asphalte  has  been  tried  on 
some  of  our  military  works,  and  has  given  thus  far  promise 
of  a  successful  issue. 

523.  Stucco  exposed  to  weather  is  sometimes  covered  with 
•         paint,  or  other  mixtures,  to  give  it  durability.    Coal  tar  has 

been  tried,  but  without  success  in  our  climate.  M.  Raucourt 
de  Charleville,  in  his  work  Traite  des  Mortiers^  gives  the 
following  compositions  for  protecting  exposed  stuccoes,  which 
he  states  to  succeed  well  in  all  climates.  For  important  work, 
three  parts  of  linseed  oil  boiled  with  one-sixth  of  its  weight 
of  litharge,  and  one  part  of  wax.  For  common  works,  one 
part  of  linseed  oil,  one-tenth  of  its  weight  of  litharge,  and 
two  or  three  parts  of  resin. 

The  surfaces  must  be  thoroughly  dry  before  applying  the 
compositions,  which  should  be  laid  on  hot  with  a  brush. 

524.  Repairs  of  Masonry.  In  effecting  repairs  in  mason- 
ry, Avhen  new  work  is  to  be  connected  with  old,  the  mortar 
of  the  old  should  be  thoroughly  cleaned  olf  wherever  it  is  in- 
jured along  the  surface  where  the  junction  is  effected,  and 
the  surface  thoroughly  wet.  The  bond  and  other  arrange- 
ments will  depend  upon  the  circumstances  of  the  case ;  the 
surfaces  connected  should  be  fitted  as  accurately  as  practical 
ble,  so  that  by  using  but  little  mortar,  no  disunion  may  take 
place  from  settling. 

525.  An  expedient,  very  fertile  in  its  applications  to  hy- 
draulic constructions,  has  been  for  some  years  in  use  among 
tlie  French  engineers,  for  stopping  leaks  in  walls  and  renew- 


rOUKDATIONS. 


261 


ing  the  beds  of  foundations  wbicli  have  yielded,  or  have  been 
otherwise  removed  by  the  action  of  water.  It  consists  in  in- 
jecting  hydraulic  cement  into  the  parts  to  be  filled,  through 
holes  drilled  through  the  masonry,  by  means  of  a  strong  sy- 
ringe. The  instruments  used  for  this  purpose  (Fig.  83  a)  are 
usually  cylinders  of  wood,  or  of  cast  iron  ;  the  bore  uniform, 
except  at  the  end,  which  is  terminated  with  a  nozle  of  the 
usual  conical  form ;  the  piston  is  of  wood,  and  is  driven  down 
by  a  heavy  mallet.  In  using  the  syringe,  it  is  adjusted  to 
the  hole ;  the  hydraulic  cement  in  a  semi-lluid  state  poured 


Fig.  83  a— Represents  the  arrangements  for  in- 
jecting hydraulic  cement  under  a  wall. 

A,  section  of  the  wall  with  vertical  holes  c,  c 
drilled  through  it. 

B,  syringe  and  piston  for  injecting  the  cement 
into  the  space  G  under  the  wall. 


into  it ;  a  wad  of  tow,  or  a  disk  of  leather  being  introduced 
on  top  before  inserting  the  piston.  The  cement  is  forced  in 
by  repeated  blows  on  the  piston. 

526.  A  mortar  of  hydraulic  lime  and  fine  sand  has  been 
used  for  the  same  purpose  ;  the  lime  being  ground  fresh  from 
the  kiln,  and  used  before  slaking,  in  order  that  by  the  in- 
crease of  volume  which  takes  place  from  slaking,  it  might  fill 
more  compactly  all  interior  voids.  The  use  of  unslaked  lime 
has  received  several  ingenious  applications  of  this  character  ; 
its  after  expansion  may  prove  injurious  when  confined.  The 
use  of  sand  in  mortar  for  injections  has  by  some  engineers 
been  condemned,  as  from  the  state  of  fluidity  in  whi'ch  the 
mortar  must  be  used,  it  settles  to  the  bottom  of  the  syringe, 
and  thus  prevents  the  formation  of  a  homogeneous  mass. 

527.  Effects  of  Temperature  on  Masonry.  Frost  is  the 
most  powerful  destuctive  agent  against  which  the  engineeer 
has  to  guard  in  constructions  of  masonry.     During  severe 


262 


CIVIL  ENGINEEBING. 


winters  in  the  northern  parts  of  our  country,  it  has  been  as- 
certained, by  observation,  that  the  frost  will  penetrate  earth 
in  contact  with  walls  to  depths  exceeding  ten  feet ;  it  there- 
fore becomes  a  matter  of  the  first  importance  to  use  every 
practicable  means  to  drain  thoroughly  all  the  ground  in  con- 
tact with  masonry,  to  whatever  depths  the  foundations  may 
be  sunk  below  the  surface;  for  if  this  precaution  be  not 
taken,  accidents  of  the  most  serious  nature  may  happen  to  the 
foundations  from  the  action  of  the  frost.  If  water  collects  in 
any  quantity  in  the  earth  around  the  foundations,  it  may  be 
necessary  to  make  small  covered  drains  under  them  to  con- 
vey it  off,  and  to  place  a  stratum  of  loose  stone  between  the 
sides  of  the  foundations  and  the  surrounding  earth  to  give  it 
a  free  downward  passage. 

It  may  be  laid  down  as  a  maxim  in  building,  that  mortar 
which  is  exposed  to  the  action  of  frost  before  it  has  set,  will 
he  so  much  damaged  as  to  impair  entirely  its  jprojperties. 
This  fact  places  in  a  stronger  light  what  has  already  been  re- 
marked, on  the  necessity  of  laying  the  foundations  and  the 
structure  resting  on  them  in  hydraulic  mortar,  to  a  height  of 
at  least  three  feet  above  the  ground  ;  for,  although  the  mortar 
of  the  foundations  might  be  protected  from  the  action  of  the 
frost  by  the  earth  around  them,  the  parts  immediately  above 
would  be  exposed  to  it,  and  as  those  parts  attract  the  mois- 
ture from  the  ground,  the  mortar,  if  of  common  lime,  would 
not  set  in  time  to  prevent  the  action  of  the  frosts  of  winter. 

In  heavy  walls  the  mortar  in  the  interior  will  usually  be  se- 
cured from  the  action  of  the  frost,  and  masonry  of  this  char- 
acter might  be  carried  on  until  freezing  weather  commences  ; 
but  still  in  all  important  works  it  will  be  by  far  the  safer 
course  to  suspend  the  construction  of  masonry  several  weeks 
before  the  ordinary  period  of  frost. 

During  the  heats  of  summer,  the  mortar  is  injured  by  a 
too  rapid  drying.  To  prevent  this  the  stone,  or  brick,  should 
he  thoroughly  moistened  before  being  laid ;  and  afterwards, 
if  the  weather  is  very  hot,  the  masonry  shoidd  be  hejpt  wet  until 
the  mortar  gives  indications  of  setting.  The  top  course  should 
always  be  well  moistened  by  the  workmen  on  quitting  their 
work  for  any  short  period  during  very  warm  weather. 

The  effects  produced  by  a  high  or  low  temperature  on  mor- 
tar in  a  green  state  are  similar.  In  the  one  case  the  freezing 
of  the  water  prevents  a  union  between  the  particles  of  tlie 
lime  and  sand ;  and  in  the  other  the  same  arises  from  the 
water  being  rapidly  evaporated.  In  both  cases  the  mortar 
when  it  has  set  is  weak  and  pulverulent. 


CHAPTEE  lY. 


FKAMmG. 

528.  Framing  is  the  art  of  arranging  beams  of  solid  mate- 
rials for  the  various  purposes  to  which  they  are  applied  in 
structures.  A  frame  is  any  arrangement  of  beams  made  for 
sustaining  strains. 

529.  That  branch  of  framing  which  relates  to  the  combina- 
tions of  beams  of  timber  is  denominated  Carpentry. 

530.  Timber  and  iron  are  the  only  materials  in  common 
use  for  frames,  as  they  are  equally  suitable  to  resist  the  vari- 
ous strains  to  be  met  with  in  structures.  Iron,  independently 
of  offering  greater  resistance  to  strains  than  timber,  possesses 
the  further  advantage  of  being  susceptible  of  receiving  the 
most  suitable  forms  for  strength  without  injury  to  the  mate- 
rial ;  while  timber,  if  wrought  into  the  best  forms  for  the 
object  in  view,  may,  in  some  cases,  be  greatly  injured  in 
strength. 

531.  The  object  to  be  attained  in  framing  is  to  give,  by  a 
suitable  combination  of  beams,  the  requisite  degree  of  strength 
and  stiffness  demanded  by  the  character  of  the  structure, 
united  with  a  lightness  and  an  economy  of  material  of  which 
an  arrangement  of  a  massive  kind  is  not  susceptible.  To 
attain  this  end,  the  beams  of  the  frame  must  be  of  such  forms, 
and  be  so  combined  that  they  shall  not  only  offer  the  greatest 
resistance  to  the  efforts  they  may  have  to  sustain,  but  shall 
not  change  their  relative  positions  from  the  effect  of  these 
efforts. 

532.  The  forms  of  the  beams  will  depend  upon  the  kind 
of  material  used,  and  the  nature  of  the  strain  to  which  it 
may  be  subjected,  whether  of  tension,  compression,  or  a  cross 
strain. 

533.  The  general  shape  given  to  the  frame,  and  the  com- 
binations of  the  beams  for  this  purpose,  will  depend  upon 
the  object  of  the  frame  and  the  directions  in  which  the  efforts 
act  upon  it. 

In  frames  of  timber,  for  example,  the  cross  sections  of  each 
beam  are  generally  uniform  throughout,  these  sections  being 
either  circular,  or  rectangular,  as  these  are  the  only  simple 


264 


CIVIL  ENGINEEEING. 


forms  which  a  beam  can  receive  without  injury  to  its  strength. 
In  frames  of  cast-iron,  each  beam  may  be  cast  into  the  most 
suitable  form  for  thestrerigth  required,  consideriug  the  econo- 
my of  the  material. 

534.  In  combining  the  beams,  whatever  may  be  the  gen- 
eral shape  of  the  frame,  the  parts  which  comjDose  it  must,  as 
far  as  practicable,  present  triangular  figures,  each  side  of  the 
triangles  being  formed  of  a  single  beam  ;  the  connection  of 
the  beams  at  the  angular  points,  termed  the  joints,  being  so 
arranged  that  no  yielding  can  take  j^lace.  In  all  combina- 
tions, therefore,  in  which  the  principal  beams  form  polygonal 
figures,  secondary  beams  must  be  added,  either  in  the  direc- 
tions of  the  diagonals  of  the  polygon,  or  so  as  to  connect  each 
pair  of  beams  forming  an  angle  of  the  polygon,  for  the  pur- 
pose of  preventing  any  change  of  form  of  the  figure,  and  of 
giving  the  frame  the  requisite  stiffness.  These  secondary 
pieces  receive  the  general  appellation  of  hraces.  AVhen  they 
sustain  a  strain  of  compression  they  are  termed  sti'uts  ;  when 
one  of  extension,  ties. 

535.  As  one  of  the  objects  of  a  frame  is  to  transmit  the 
strain  it  directly  receives  to  firm  points  of  support,  the  beams 
of  which  it  is  formed  should  be  so  combined  that  this  may 
be  done  in  the  w^ay  which  shall  have  the  least  tendency  to 
change  the  shape  of  the  frame  and  to  fracture  the  beams. 

536.  The  points  of  support  of  a  frame  may  be  either 
above  or  below  it.  In  the  former  case,  the  frame  will  con- 
sist of  a  suspended  system,  in  which  the  polygon  will  assume 
a  position  of  stable  equilibrium,  its  sides  being  subjected  to  a 
strain  of  extension.  In  the  latter  case  the  frame,  if  of  a 
polygonal  form,  must  satisfy  the  essential  conditions  already 
enumerated,  in  order  that  its  state  of  equilibrium  shall  be 
stable. 

537.  The  object  of  the  structure  will  necessarily  decide 
the  general  shape  of  the  frame,  as  well  as  the  direction  of 
the  strains  to  which  it  will  be  subjected.  An  examination, 
therefore,  of  the  frames  adapted  to  some  of  the  more  usual 
structures  will  be  the  best  course  for  illustrating  both  the 
preceding  general  principles  and  the  more  ordinary  combina- 
tions of  me  beams  and  joints. 

538.  Frames  for  Cross  Stradns.  The  parts  of  a  frame 
which  receive  a  cross  strain  may  be  horizontal,  as  the  beams, 
or  joists  of  a  floor;  or  inclined,  as  the  beams,  or  rafters 
which  form  the  inclined  sides  of  the  frame  of  a  roof.  The 
pressure  producing  the  cross  strain  may  either  be  uniformly 
distributed  over  the  beams,  as  in  the  cases  just  cited,  or  it 


\ 


FRAMING. 


265 


may  act  only  at  one  point,  as  in  the  case  of  a  weight  laid 
upon  the  beam. 

In  all  of  these  cases  the  extremities  of  the  beam  should  be 
firmly  fixed  against  immovable  points  of  support ;  the  longer 
side  of  the  rectangular  section  of  the  beam  should  be  par- 
allel to  the  direction  of  the  strain,  as  this  is  the  best  position 
for  strength. 

If  the  distance  between  the  points  of  support,  or  the  l^ear- 
ing^  be  not  great,  the  framing  may  consist  simply  of  a  row 
of  parallel  beams  of  such  dimensions,  and  placed  so  far  asun- 
der as  the  strain  borne  may  require.    When  the  beams  are 


Fig.  84 — Represents  a  cross  section  of  horizontal  beams  a,  a  braced 
by  diagonal  battens  b. 


narrow,  or  the  depth  of  the  rectangle  considerably  greater 
than  the  breadth  (Fig.  84),  short  struts  of  battens  may  be 
placed  at  intervals  between  each  pair  of  beams,  in  a  diagonal 
direction,  uniting  the  bottom  of  the  one  with  the  top  of  the 
other,  to  prevent  the  beams  from  twisting,  or  yielding  late- 
rally. This  also  increases  the  stiffness  of  the  structure  by 
distributing  the  strains. 

539.  When  the  bearing  and  strain  are  so  great  that  a  sin- 
gle beam  will  not  present  sufficient  strength  and  stiffness,  a 
combination  of  beams,  termed  a  huilt  heam^  which  may  be 
solid,  consisting  of  several  layers  of  timber  laid  in  juxtapo- 
sition, and  firmly  connected  together  by  iron  bolts  and  straps 
— or  oj)en,  being  formed  of  two  beams,  with  an  interval  be- 
tween them,  so  connected  by  cross  and  diagonal  pieces,  that 
a  strain  upon  either  the  upper  or  lower  beam  will  be  trans- 
mitted to  the  other,  and  the  whole  system  act  under  the  effect 
of  the  strain  like  a  solid  beam. 

540.  Solid  built  Beams.  In  f ranging  solid  built  beams, 
the  pieces  in  each  course  (Fig.  85)  are  laid  abutting  end  to 

Fig.  85 — Represents  a  solid  built  beam 
of  three  courses,  the  pieces  of  each 
course  breaking  joints  and  confined 
by  iron  hoops. 

end  with  a  square  joint  between  them,  the  courses  breaking 
joints  to  form  a  strong  bond  between  them.  The  courses 
are  firmly  connected  either  by  iron  bolts,  formed  with  a 
screw  and  nut  at  one  end  to  bring  the  courses  into  close  con- 


266 


CrVIL  ENGINEERING. 


tact,  or  else  by  iron  bands  driven  on  tight,  or  by  iron  stirrups 
(Fig.  86)  suitably  arranged  with  screw  ends  and  nuts  for  the 
same  purpose. 


Pig.  86— Represents  an  iron  stirrup  or  hoop  with  nuts  or  female  screws 
c  which  confine  the  crosa  piece  of  the  stirrup  6. 


When  the  strain  is  of  such  a  character  that  the  courses 
would  be  liable  to  work  loose  and  slide  along  their  joints,  the 
beams  of  the  different  courses  may  be  made  with  shallow  in- 
dentations (Figs.  87,  88),  accurately  fitting  into  each  other  ; 


Fig.  87 — Represents  a  solid  built  beam 
of  three  courses  arranged  with  in- 
dents and  confined  by  iron  hoops. 


F=I 


XT 


Fig.  88 — Represents  a  solid  built  beam,  the  top  part  being  of  two  pieces  6,  h  which  abut 
against  a  broad  flat  iron  bolt  a,  termed  a  king  bolt. 

or  shallow  rectangular  notches  (Fig.  89)  may  be  cut  across 
each  beam,  being  so  placed  as  to  receive  blocks,  or  Jceys  of 


r 

— o- 



 ra- 

— r 

J — 

— ss  f 

1  

Fig.  89 — Represents  a  solid  built  beam 
with  kej  s  6,  b  of  hard  wood  between 
the  courses. 


hard  wood.    The  keys  are  sometimes  made  of  two  wedge- 


1 


I 


Fig.  90— Represents  the  keys  in  the  form  of  double, 
ox  folding  wedges  o,  b  let  into  a  shallow  notch 
in  the  beam  c. 


shaped  pieces  (Fig.  90),  for  the  purpose  of  causing  them  to 
fit  the  notches  more  closely,  and  to  admit  of  being  driven 
tight  upon  any  shrinkage  of  the  woody  fibre. 

The  joints  between  the  courses  may  be  left  slightly  open 
without  impairing  in  an  appreciable  degree  the  strength  of 
the  combination.    This  is  a  good  method  in  beams  exposed  o 


FRAMING. 


267 


moisture,  as  it  allows  of  evaporation  from  the  free  circulation 
of  the  air  through  the  joints.  Felt,  or  stout  paper  saturated 
with  mineral  tar,  has  been  recommended  to  secure  the  joints 
from  the  action  of  moisture.  The  prepared  material  is  so 
placed  as  to  occupy  the  entire  surface  of  the  joint,  and  the 
whole  is  well  screwed  together. 

541.  Joints.  A  joint  is  the  surface  between  two  pieces 
which  come  in  contact  with  each  other,  and  which  are  connected 
together.  The  form  and  arrangement  of  joints  will  depend 
upon  the  relative  position  of  the  beams  joined,  and  the  object 
of  the  joint. 

In  all  arrangements  of  joints,  the  axes  of  the  beams  con- 
nected should  lie  in  the  same  plane  in  which  the  strain  upon 
the  frame  acts ;  and  the  combination  should  be  so  arranged 
that  the  parts  will  accurately  fit  when  the  frame  is  put  to- 
gether, and  that  any  portion  may  be  displaced  without  dis- 
connecting the  rest.  The  simplest  forms  most  suitable  to  the 
object  in  view  will  usually  be  found  to  be  the  best. 

In  adjusting  the  surfaces  of  the  joints  an  allowance  should 
be  made  for  any  settling  in  the  frame  which  may  arise  either 
from  the  shrinking  of  the  timber  in  seasoning  while  in  the 
frame,  or  from  the  fibres  yielding  to  the  action  of  the  strain. 
This  is  done  by  leaving  sufticient  play  in  the  joints  when  the 
frame  is  first  set  up,  to  admit  of  the  parts  coming  into  per- 
fect contact  when  the  frame  has  attained  its  final  settling. 
Joints  formed  of  plane  surfaces  present  more  difticulty  in 
this  respect  than  curved  joints,  as  the  bearing  surfaces  in  the 
latter  case  will  remain  in  contact  should  any  slight  change 
take  place  in  the  relative  positions  of  the  beams  from  settling ; 
whereas  in  the  former  a  slight  settling  might  cause  the  strains 
to  be  thrown  upon  a  corner,  or  edge  of  the  joint,  by  which 
the  bearing  surfaces  might  be  crushed,  and  the  parts  of  the 
framework  wrenched  asunder  from  the  leverasce  which  such 
a  circumstance  might  occasion. 

The  surface  of  a  joint  subjected  to  pressure  should  be  as 
great  as  practicable,  to  secure  the  parts  in  contact  from  being 
crushed  by  the  strain ;  and  the  surface  should  be  nearly  per- 
pendicular to  the  direction  of  the  strain  to  prevent  sliding. 

A  thin  plate  of  iron,  or  lead,  may  be  inserted  between  the 
surfaces  of  joints  where,  from  the  magnitude  of  the  strain, 
one  of  them  is  liable  to  be  crushed  by  the  other,  as  in  the 
case  of  the  end  of  one  beam  resting  upon  the  face  of  another. 

542.  Folding  wedges,  and  pins,  or  tree-nails^  of  hard  wood, 
are  used  to  bring  the  surfaces  of  joints  firmly  to  their  bear- 
ings, and  retain  the  parts  of  the  frame  in  their  places.  The 


268 


CIVIL  ENGrNEEKmG. 


wedges  are  inserted  into  square  holes,  and  the  pins  into  auger- 
holes  made  through  the  parts  connected.  As  the  object  of 
these  accessories  is  sira2)ly  to  bring  the  parts  connected  into 
close  contact,  they  should  be  carefully  driven,  in  order  not  to 
cause  a  strain  that  might  crush  the  fibres. 

To  secure  joints  subjected  to  a  heavy  strain,  bolts,  straps, 
and  hoops  of  wrought  iron  are  used.  These  should  be  placed 
in  the  best  direction  to  counteract  the  strain  and  prevent  the 
parts  from  separating ;  and  wherever  the  bolts  are  requisite 
they  should  be  inserted  at  those  points  which  will  least  weaken 
the  joint. 

543.  Joints  of  Beams  united  end  to  end.  When  the  axes 
of  the  beams  are  in  the  same  right  line,  the  form  of  the  joint 
will  depend  upon  the  direction  of  the  strain.  If  the  strain  is 
one  of  compression,  tha  ends  of  the  beams  may  be  united  by 
a  square  joint  perpendicular  to  their  axes,  the  joint  being 
secured  (Fig.  91)  by  four  short  pieces  so  placed  as  to  embrace 


i 


Fig.  91 — Eepresents  the  manner  in  which  the  end  joint  of  two  beams  a  and  6  is  fished  or 
Becuied  by  side  pieces  c  and  d  bolted  to  them. 


the  ends  of  the  beams,  and  being  fastened  to  the  beams  and 
to  each  other  by  bolts.  This  arrangement,  termed  fisldng  a 
heam,  is  used  only  for  rough  work.  It  may  also  be  used 
when  the  strain  is  one  of  extension;  in  which  case  the  short 
pieces  (Fig.  92)  may  be  notched  upon  the  beams,  or  else  keys 


Pig.  92 — Eepresents  a  fished  joint  in  which  the  side  pieces  c  and  d  are  either  let  into  the 
beams  or  secured  by  keys  e,  e. 


of  hard  wood,  inserted  into  shallow  notches  made  in  the  beams 
and  short  pieces,  may  be  employed  to  give  additional  security 
to  the  joint. 

A  joint  termed  a  scarf  m?iy  be  used  for  either  of  the  fore- 
going purposes.    This  joint  may  be  formed  either  by  halving 


FRAMING. 


269 


i 


1 


Fig.  93 — Eepresents  a  scarf  joint  secured  by  iron  plates  c,  c,  keys,  d,  c?,  and  bolts, 

the  beams  on  each  other  near  their  ends  (Fig.  93),  and  se- 
cnring  the  joints  by  bolts,  or  straps  ;  or  else  by  so  arranging 
the  ends  of  the  two  beams  that  each  shall  fit  into  shallow 
triangular  notches  cut  into  the  other,  the  joint  being  secured 
by  iron  hoops.  This  last  method  is  employed  for  round 
timber. 

544.  When  beams  imited  at  their  ends  are  subjected  to 
a  cross  strain,  a  scarf  joint  is  generally  used,  the  under 
part  of  the  joint  being  secured  by  an  iron  plate  confined 
to  the  beams  by  bolts.  The  scarf  for  this  purpose  may 
be  formed  simply  by  halving  the  beams  near  their  ends ; 
but  a  more  usual  and  better  form  (Fig.  94)  is  to  make 


•B  a- 


3 


Fig.  94 — ^Represents  a  scarf  joint  for  a  cross  strain  secured  at  bottom  by  a  piece  of  tim- 
ber c  confined  to  the  beams  by  iron  hoops     d  and  keys  e,  e. 

the  portion  of  the  joint  at  the  top  surface  of  the  beams  per- 
pendicular to  their  axes,  and  about  one  third  of  their  depth  ; 
the  bottom  portion  being  oblique  to  the  axis,  as  w^ell  as  the 
portion  joining  these  two. 

When  the  beams  are  subjected  to  a  cross  strain  and  to  one 
of  extension  in  the  direction  of  their  axes,  the  form  of  the 
scarf  must  be  suitably  arranged  to  resist  each  of  these  strains. 
The  one  shown  in  Fiff.  95  is  a  suitable  and  usual  form  for 


Fig.  9.5 — Represents  a  scarf  joint  arranged  to  resist  a  cross  strain  and  one  of  extension.  The 
bottom  of  the  joint  :s  secured  by  an  iron  plate  confined  by  bolts.  The  folding  wedge  key 
inserted  at  c  serves  to  bring  all  the  surfaces  of  the  joints  to  their  bearings. 

these  objects.  A  folding  wedge  key  of  hard  wood  is  in- 
serted into  a  space  left  between  the  parts  of  the  joint  which 
catch  when  the  beams  are  drawn  apart.  The  key  serves  to 
bring  the  surfaces  of  the  joints  to  their  bearings,  and  to  form 
an  abutting  surface  to  resist  the  strain  of  extension.    In  this 


270 


CIVIL  ENGINEERING. 


form  of  scarf  the  surface  of  the  joint  which  abuts  against 
the  key  will  be  compressed;  the  portions  of  the  beams  just 
above  and  below  the  key  will  be  subjected  to  extension. 
These  parts  should  present  the  same  amount  of  resistance,  or 
have  an  equality  of  cross  section.  The  length  of  the  scarf 
should  be  regulated  by  the  resistance  with  which  the  timber 
employed  resists  detrusion  compared  with  its  resistance  to 
compression  and  extension. 

545.  When  the  axes  of  beams  form  an  angle  between 
them,  they  may  be  connected  at  their  ends  either  by  halving 
them  on  each  other,  or  by  cutting  a  mortise  in  the  centre 
of  one  beam  at  the  end,  and  shaping  the  end  of  the  other  to 
fit  into  it.    See  Fig.  97. 

546.  Joints  for  connecting  the  end  of  one  beam  with 
the  face  of  another.  The  joints  used  for  this  purpose 
are  termed  mortise  and  tenon  joints.  Their  form  will 
depend  upon  the  angle  between  the  axes  of  the  beams. 


pig.  96 — ^Represents  a  mortise  and  tenon 
joint  when  the  axes  of  the  beams  are  per- 
pendicular to  each  other. 

a,  tenon  on  the  beam  A. 

6,  mortise  in  the  beam  B. 

c,  pin  to  hold  the  parts  together. 


f 


When  the  axes  are  perpendicular  to  each  other,  the  mor- 
tise (Fig.  96)  is  cut  into  the  face  of  the  beam,  and  the  end 
of  the  other  beam  is  shaped  into  a  tenon  to  fit  the  mortise. 


B 

/ 

\ 

Fig.  97 — Represents  a  mortise  and  tenon 
joint  when  the  axes  of  the  beams  are 
oblique  to  each  other.  A  notch  whose 
surfaces  ab  and  he  are  at  right  angles  is 
cut  into  the  beam  B,  and  a  shadlow  mortise 
d.  is  cut  below  the  surface  he.  The  end  of 
the  beam  A  is  arranged  to  fit  the  notch  and 
mortise  in  B.  The  joint  is  secured  by  a 
Bcrew  bolt. 


When  the  axes  of  the  beams  are  oblique  to  each  other,  a 
triangular  notch  (Fig.  97)  is  usually  cut  into  the  face  of 


FRAMINa. 


271 


one  beam,  the  sides  of  the  notch  being  perpendicular  to 
each  other,  and  a  shallow  mortise  is  cut  into  the  lower 
surface  of  the  notch ;  the  end  of  the  other  beam  is  suitably 
shaped  to  fit  the  notch  and  mortise. 

The  direction  of  the  strain  and  the  effect  it  may  produce 
upon  the  joint  must  in  all  cases  regulate  its  form.  In  some 
cases  the  circular  joint  may  be  more  suitable  than  those 
forms  which  are  plane  surfaces;  in  others  a  double  tenon 
may  be  better  than  the  simple  joint. 

547.  Tie  Joints.  These  joints  are  used  to  connect  beams 
which  cross,  or  lie  on  each  other.  The  simplest  and  strong- 
est form  of  tie  joint  consists  in  cutting  a  notch  in  one  or  both 
of  the  beams  to  connect  them  securely.  But  when  the  beams 
do  not  cross,  but  the  end  of  one  rests  upon  the  other,  a  notch 
of  a  trapezoidal  form  (Fig.  98)  may  be  cut  in  the  lower  beam 


E 


I 


Fig.  98 — Eepresents  an  ordinary  dove-tail  joint  secured  by 
a  pin  at  c 


to  receive  the  end  of  the  upper,  which  is  suitably  shaped  to 
fit  the  notch.  This,  from  its  shape,  is  termed  a  dove-tail 
joint.  It  is  of  frequent  use  in  joinery,  but  is  not  suitable 
lor  heavy  frames  where  the  joints  are  subjected  to  consider- 
able strains,  as  it  soon  becomes  loose  from  the  shrinking  of 
the  timber. 

548.  Open  built  Beams.  In  framing  open  built  beams, 
the  principal  point  to  be  kept  in  view  is  to  form  such  a  con- 
nection between  the  upper  and  lower  solid  beams,  that  they 
shall  be  strained  uniformly  by  the  action  of  a  strain  at  any 
point  between  the  bearings.  This  may  be  effected  in  various 
ways,  (Fig.  99.)    The  upper  and  lower  beams  may  consist 


Fig.  99— Eepresents  an  open 
built  beam ;  A  and  B  are 
the  top  and  bottom  rails  or 
Btrings;  a,  a,  cross  pieces, 
either  single  or  in  pairs;  6, 
diagonal  braces  in  pairs;  c, 
single  diagonal  braces. 


either  of  single  beams  or  of  solid  built  beams;  these  are  con- 
nected at  regular  intervals  by  pieces  at  right  angles  to  them, 
between  which  diagonal  pieces  are  placed.    By  this  arrange- 


272 


CIVTL  ENGIETEEKING. 


ment  the  relative  position  of  all  the  parts  of  the  frame  will 
be  preserved,  and  the  strain  at  any  point  will  be  brought  to 
bear  upon  the  intermediate  points. 

549.  Framing  for  intermediate  Supports.  Beams  of 
ordinary  dimensions  may  be  used  for  wide  bearings  when 
intermediate  supports  can  be  procured  between  the  extreme 
points. 

The  simplest  and  most  obvious  method  of  effecting  this  is 
to  place  upright  beams,  termed  jorojps^  or  shores^  at  suitable 
intervals  under  the  supported  beam. 

When  the  props  would  interfere  with  some  other  arrange- 
ment, and  points  of  support  can  be  procured  at  the  extremi- 
ties below  those  on  which  the  beam  rests,  inclined  struts  (Fig. 
100)  may  be  used.  The  struts  must  have  a  suitably  formed 
step  at  the  foot,  and  be  connected  at  top  with  the  beam  by  a 
suitable  joint. 

In  some  cases  the  bearing  may  be  diminished  by  placing 


on  the  points  of  support  short  pieces,  termed  corbels  (Fig.  101), 
and  supporting  these  near  their  ends  by  struts. 


Fig.  101— Hepresents  a 
hoi-izontal  beam  c  sup- 
ported by  vertical 
popts  (7,  a.  with  corbel 
pieces  d.cl  and  inclined 
struts  e  to  diminish 
the  bearing. 


In  other  cases  a  portion  of  the  beam,  at  the  middle,  may 
be  strengthened  by  placing  under  it  a  short  beam,  called  a 


Fig.  102— Represents  a 
horizontal  beam  c, 
ptrcnpthened  by  a 
strainintr  beam  /  and 
inclined  struts  «,  e. 


straining  learn  (Fig.  102),  against  the  ends  of  which  the 
struts  abut. 


FRAMING. 


273 


Whenever  the  bearing  may  require  it  the  two  preceding 
arrangements  (Fig.  103)  may  be  used  in  connection. 


Fig.  103— Eepresents  a  combination  of  Figs.  101  and  102. 


In  all  combinations  with  struts,  a  lateral  thrust  will  be 
thrown  on  the  point  of  support  where  the  foot  of  the  strut 
rests.  This  strain  must  be  provided  for  in  proportioning  the 
supports. 

550.  When  intermediate  supports  can  be  procured  only 
above  the  beam,  an  arrangement  must  be  made  which  shall 
answer  the  purpose  of  sustaining  the  beam  at  its  interme- 
diate points  by  suspension.  The  combination  will  depend 
upon  the  number  of  intermediate  points  required. 

When  the  beam  requires  to  be  supported  only  at  the  mid- 
dle, it  may  be  done  as  shown  in  Fig.  104.  If  the  suspending 
piece  be  of  iron,  it  must  be  arranged  at  one  end  with  a  screw 
and  nut.  When  the  support  is  of  timber,  a  single  beam, 
called  a  king  jpost,  (Fig.  104,)  may  be  used,  against  the  head 


Fig.  104 — Represents  a 
horizontal  beam  c 
supported  in  its  mid- 
dle by  a  king  post  g 
suspended  from  the 
struts  €,  e. 


of  which  the  two  inclined  pieces  may  abut ;  the  foot  of  the 
post  is  connected  with  the  beam  by  a  bolt,  an  iron  stirrup,  or 
a  suitable  joint.  Instead  of  the  ordinary  king  post,  two 
beams  may  be  used ;  these  are  placed  opposite  to  each  other 
and  bolted  together,  embracing  between  them  the  supported 
beam  and  the  heads  of  the  inclined  beams  which  fit  into  shal- 
low notches  cut  into  the  supporting  beams.   Pieces  arranged 


274 


CIVIL  ENGINEERING. 


in  this  manner  for  suspending  portions  of  a  frame  receive  the 
name  of  suspension jpieceSj  or  bridle  jpieces. 

When  two  intermediate  points  of  support  are  required,  they 
may  be  obtained  as  shown  in  Fig.  105.     The  suspension 


Fig.  105 — Repreaents  a  beam  c 

supported  at  two  points  by 
posts  g,  g  suspended  from  the 
struts  e,  e  and  straining  beam 
A. 


pieces  in  this  case  may  be  either  posts,  termed  queen  posts, 
arranged  like  a  king  post,  iron  rods,  or  bridle  pieces.  This 
combination  may  be  used  for  very  wide  bearings,  (Fig.  106,) 
b}^  suitably  increasing  the  number  of  inclined  pieces  and 
straining  beam. 


551.  Experiments  on  the  Strength  of  Frames.  Experi- 
mental researches  on  this  point  have  been  mostly  restricted 
to  those  made  with  models  on  a  comparatively  small  scale, 
owning  to  the  expense  and  difficulty  attendant  upon  experi- 
ments on  frames  having  the  form  and  dimensions  of  those 
employed  in  ordinary  structures. 

Among  the  most  remarkable  experiments  on  a  large  scale, 
are  those  made  by  order  of  the  French  government  at  Lori- 
ent,  under  the  direction  of  M.  Kiebell,  the  superintending 
engineer  of  the  port,  and  published  in  the  Annates  Mari- 
times  et  Coloniales,  Feb.  and  Nov.,  1837. 

The  experiments  w^ere  made  by  first  setting  up  the  frame 
to  be  tried,  and,  after  it  had  settled  under  the  action  of  its 


FRAMING. 


275 


own  weight,  suspending  from  the  back  of  it,  by  ropes  placed 
at  equal  intervals  apart,  equal  weights  to  represent  a  load 
uniformly  distributed  over  the  back  of  the  frame. 

The  results  contained  in  the  following  table  are  from  ex- 
periments on  a  truss  (Fig.  107)  for  the  roof  of  a  ship  shed. 
The  truss  consisted  of  two  rafters  and  a  •  tie  beam,  with  sus- 


Pig.  107. 


pension  pieces  in  pairs,  and  diagonal  iron  bolts,  which  were 
added  because  it  was  necessary  to  scarf  the  tie  beam.  The 
span  of  the  truss  was  65 J  feet ;  the  rafters  had  a  slope  of  1 
perpendicular  to  4  base.  The  thickness  of  the  beams,  meas- 
ured horizontally,  was  about  2^  inches,  their  depth  about  18 
inches.  The  amount  of  the  settling  at  each  rope  was  ascer- 
tained by  fixed  graduated  vertical  rods,  the  measures  being 
taken  below  a  horizontal  line  marked  0. 


WKIGHTS  BORNE  BT  THE  TBX7SS. 


Amount  of  settling  on  the  right  of 
the  ridge  below  the  horizontal  0, 
in  inches. 


.a  <D 


a 

^8) 


Weight  nnif or mly  distributed,  1654  lbs  

Do.  do.  8680  lbs  

Do.                  do.            1654  lbs.  and  1368  lbs., sus- 
pended from  the  centre  of  the  frame  

8680  lbs.,  uniformly  distributed,  and  1368  lbs.  from  the 
centre  


0.15 
1.6 


0.15 
1.7 


0.5 
2.1 


0.15 
1.9 


0.4 
2.3 


0.15 
1.8 


0.3 
2.1 


The  following  table  gives  the  results  of  experiments  made 
on  frames  of  the  usual  forms  of  straight  and  curved  timber 
for  roof  trusses.  The  curved  pieces  were  made  of  two  thick- 
nesses, each  3^  inches.  The  numbers  in  the  fifth  column 
give  the  ratios  between  the  weight  of  the  frame  and  that  of 
the  weight  borne  by  which  the  elasticity  was  not  impaired. 


276 


CIVIL  ENGINEEKING. 


DXSCBIPTION  OF  THE  Z'BAMES. 


1 

a  0) 


2  >» 
53 

•S3 


Frame  formed  of  two  rafters  and  a  tie  beam. . 
Do.  do.  do. 

and  suspension  pieces  in  pairs,  (Fig.  108)  , 
Frame  of  a  segment  arch  confined  by  a  tie 

beam,  (Fig.  10<»)  

Do.  do.  do. 

with  suspension  pieces  in  pairs,  (Fig.  110). . 
Frame  of  a  segment  arch  with  rafters  con 
fined  at  their  foot  by  a  tie  piece,  (Fig.lll). . 
Frame  of  a  full  centre  arch  confined  by  a  tie 

beam  

Do.                do.  do. 
Vnth.  suspension  pieces  in  pairs  


25  ft. 


8ft. 


8.5  in, 


3.1  in. 


54  ft. 


lift. 


12  in. 


Tin, 


50  ft. 


25  ft. 


14.30 
8.88 
3.85 
2.82 
3.91 
1.00 
0.91 


2o00 
2770 
6520 
9500 
6111 
4336 
7328 


3916 
5520 
12240 
18077 
21896 
6161 
8153 


Fig.  108. 


Pig.  110. 


p— ^  ^-E  

FEAMIUG. 


277 


Fig.  IIL 


Fig.  112— Represents  a  wooden  arch  A  formed  of  a  solid  bnilt  beam  of  three 
courses  which  support  the  beams  c,  c  by  the  posts  g  which  are  formed 
of  pieces  in  pairs. 

&,  6,  inclined  struts  to  strengthen  the  arch  by  relieving  it  of  a  part  of  the 
load  on  the  beams  c,  c. 


A 


Fig.  113 — ^Represents  a  wooden  arch  of  a  solid  built  beam  A  which  supports 
the  horizontal  beam  B  by  means  of  the  posts  a,  a.  The  arch  is  let  into 
the  beam  B,  which  acts  as  a  tie  to  confine  its  extremities. 

552.  "Wooden  arches  may  also  be  formed  by  fastening  to- 
gether several  courses  of  boards,  giving  the  frame  a  polygo- 
nal form,  (Fig.  114,)  corresponding  to  the  desired  curvature, 
and  then  shaping  the  outer  and  inner  edges  of  the  arch  to  the 
proper  curve.    Each  course  is  formed  of  boards  cut  into 


278^ 


CIVIL  ENGINEEEING. 


Fig.  114 — Represents  an  elevation  A  of  a 
wooden  arch  formed  of  short  pieces  a,  b 
•which  abut  end  to  end  and  break  joints. 

B  represents  a  perspective  view  of  this  com- 
bination,  showing  the  manner  in  which 
the  parts  are  keyed  together. 


sharp  lengths,  depending  on  the  curvature  required  ;  these 
pieces  abut  end  to  end,  the  joints  being  in  the  direction  of 
the  radii  of  curvature,  and  the  pieces  composing  the  different 
courses  break  joints  with  each  other.  The  courses  may  be 
connected  either  by  jibs  and  keys  of  hard  wood,  or  by  iron 
bolts.  This  method  is  very  suitable  for  all  light  framework 
where  the  pressure  borne  is  not  great. 

Wooden  arches  are  chiefly  used  for  bridges  and  roofs. 
They  serve  as  intermediate  points  of  support  for  the  framing 
on  which  the  roadway  rests  in  the  one  case,  and  the  roof 
covering  in  the  other.  In  bridges  the  roadway  may  lie  either 
above  the  arch,  or  below  it ;  in  either  case  vertical  posts, 
iron  rods,  or  bridles  connect  the  horizontal  beams  with  the 
arch. 

553.  The  greatest  strain  in  wooden  arches  takes  place 
near  the  springing  line ;  this  part  should,  therefore,  when 
practicable,  be  relieved  of  the  pressure  that  it  would  directly 
receive  from  the  beams  above  it  by  inclined  struts,  so  arranged 
as  to  throw  this  pressure  upon  the  lateral  supports  of  the 
arch. 

The  pieces  which  compose  a  wooden  arch  may  be  bent  into 
any  curve.  The  one,  however,  usually  adopted  is  an  arc  of  a 
circle,  as  the  most  simple  for  the  mechanical  construction  of 
the  framing,  and  presenting  all  desirable  strength. 


CHAPTEE  Y. 


BEIDGES. 

L  Classification.  II.  Stone  Bridges.  III.  Wooden 
Bridges.  IY.  Cast-Ikon  Bridges.  Y.  Wrought- 
Iron  Truss  Bridges.  YI.  Tubular  Bridges.  YII. 
Suspension  Bridges.  YIII.  Swing  Bridges.  IX. 
Aqueduct  Bridges. 

1. 

classification. 

554.  A  hridge  is  a  structure  for  supporting  a  roadway  over 
a  body  or  stream  of  water,  or  over  a  depression  in  the  earth. 

If  the  structure  is  over  a  depression  in  which  there  is 
usually  no  water,  it  is  called  a  viaduct. 

If  the  structure  supports  a  water-way,  it  is  called  an  aque- 
duct, and  if  the  aqueduct  is  over  a  river,  it  is  sometimes 
called  an  aqueduct-bridge. 

Bridges  may  be  classed  according  to  their  mechanical 
features;  in  which  case  we  have — 

1.  Arches. 

2.  Trussed  bridges. 

3.  Tubular  bridges. 

4.  Suspension  bridges. 

They  may  also  be  classed  according  to  the  materials  which 
compose  them ;  as  Stone,  Wood,  and  Iron. 

The  former  is  more  convenient  for  the  purposes  of  analy- 
sis, but  the  latter  will  be  used  in  this  work. 

11. 

STONE  bridges.  " 

555.  A  stone  bridge  consists  of  a  roadway  which  rests  upon 
one  or  more  arches,  usually  of  a  cylindrical  form,  the  abut- 
ments and  piers  of  the  arches  being  of  sufficient  height  and 
strength  to  secure  them  and  the  roadway  from  the  effects  of 
an  extraordinary  rise  in  the  water-course. 


280 


CIVIL  ENGINEEEING. 


556.  The  general  location  of  a  bridge  will  depend  npon 
the  approaches,  and  the  particular  locality  may  be  modified 
by  the  character  of  the  banks,  the  soil  or  subsoil,  and  the 
bends  in  the  stream.  High  embankments  and  deep  excava- 
tions will  naturally  be  avoided,  if  possible.  The  faces  of  the 
piers  and  abutments  should  be  nearly  or  quite  parallel  to  the 
thread  of  the  stream. 

557.  Survey.  "With  whatever  considerations  the  locality 
may  have  been  selected,  a  careful  survey  must  be  made  not 
only  of  it,  but  also  of  the  water-course  and  its  environs  for 
some  distance  above  and  below  the  point  which  the  bridge 
will  occupy,  to  enable  the  engineer  to  judge  of  the  probable 
effects  which  the  bridge,  when  erected,  may  have  upon  the 
natural  regimen  of  the  water-course. 

The  object  of  the  survey  will  be  to  ascertain  thoroughly 
the  natural  features  of  the  surface,  the  nature  of  the  subsoil 
of  the  bed  and  banks  of  the  water-course,  and  the  character 
of  the  water-course  at  its  different  phases  of  high  and  low 
water,  and  of  freshets.  This  information  will  be  embodied 
in  a  topographical  map  ;  in  cross  and  longitudinal  sections  of 
the  water-course  and  the  substrata  of  its  bed  and  banks,  as 
ascertained  by  soundings  and  borings ;  and  in  a  descrijDtive 
memoir  which,  besides  the  usual  state  of  the  w^ater-course, 
should  exhibit  an  account  of  its  changes,  occasioned  either 
by  permanent  or  by  accidental  causes,  as  from  the  effects  of 
extraordinary  freshets,  or  from  the  construction  of  bridges, 
dams,  and  other  artificial  changes  either  in  the  bed  or  banks. 

558.  Water-way.  When  the  natural  water-way  of  a  river 
is  obstructed  by  any  artificial  means,  the  contraction,  if  con- 
siderable, will  cause  the  water,  above  the  point  where  the 
obstruction  is  placed,  to  rise  higher  than  tlie  level  of  that 
below  it,  and  produce  a  fall,  with  an  increased  velocity 
due  to  it,  in  the  current  between  the  two  levels.  These 
causes,  during  heavy  freshets,  may  be  productive  of  serious 
injury  to  agriculture,  from  the  overflowing  of  the  banks  of 
the  water-course ; — may  endanger  if  not  entirely  suspend 
navigation,  during  the  seasons  of  freshets  ; — and  expose  any 
structure  which,  like  a  bridge,  forms  the  obstruction,  to  ruin, 
from  the  increased  action  of  the  current  upon  the  soil  around 
its  foundations.  If,  on  the  contrary,  the  natural  water-way  is 
enlarged  at  the  point  where  the  structure  is  placed,  with  the 
view  of  preventing  these  consequences,  the  velocity  of  the 
current,  during  the  ordinary  stages  of  the  water,  will  be  de- 
creased, and  this  will  occasion  deposits  to  be  formed  at  the 
point,  which,  by  gradually  filling  up  the  bed,  might,  on  a 


LOCATION  OF  BRIDGES. 


281 


sudden  rise  of  the  water,  prove  a  more  serious  obstruction 
than  the  structure  itself ;  particular!}^  if  the  main  bodj  of  the 
water  should  happen  to  be  diverted  by  the  deposit  from  its 
ordinary  channels,  and  form  new  ones  of  greater  depth 
around  the  foundations  of  the  structure. 

The  water-way  left  by  the  structure  should,  for  the  reasons 
above,  be  so  regulated  that  no  considerable  change  shall  be 
occasioned  in  the  velocity  of  the  current  through  it  during  the 
most  unfavorable  stages  of  the  water. 

559.  For  the  purpose  of  deciding  upon  the  most  suitable 
velocity  for  the  current  through  the  contracted  water-way 
formed  by  the  structure,  the  velocity  of  the  current  and  its 
effects  upon  the  soil  of  the  banks  and  bed  of  the  natural  water- 
way should  be  carefully  noted  at  those  seasons  when  the  water 
is  highest ;  selecting,  in  preference,  for  these  observations,  those 
points  above  and  below  the  one  which  the  bridge  is  to  occupy, 
where  the  natural  water-way  is  most  contracted. 

560.  The  velocity  of  the  current  at  any  point  may  be  ascer- 
tained by  the  simple  process  of  allowing  a  light  ball,  or  float 
of  some  material,  like  white  wax,  or  camphor,  whose  specific 
gravity  is  somewhat  less  than  that  of  water,  to  be  carried  along 
by  the  current  of  the  middle  thread  of  the  water-course,  and 
noting  the  time  of  its  passage  between  two  fixed  stations. 

561.  From  the  velocity  at  the  surface,  ascertained  in  this 
way,  the  average,  or  mean  velocity  of  the  water,  which  flows 
through  the  cross-section  of  any  water-way  between  the  sta- 
tions where  the  observations  are  taken,  is  nearly  four-fifths  of 
the  velocity  at  the  surface. 

Having  the  mean  velocity  of  the  natural  water-way,  that  of 
the  artificial  water-way  will  be  obtained  from  the  following 
expression, 

v  —  m  —  Y, 
s  ' 

in  which  s  and  v  represent,  respectively,  the  area  and  mean 
velocity  of  the  artificial  water-way ;  S  and  Y,  the  same  data  of 
the  natural  water-way ;  and  m  a  constant  quantity,  which,  as 
determined  from  various  experiments,  may  be  represented  by 
the  mixed  number  1,097. 

With  regard  to  the  effect  of  the  increased  velocity  on  the 
bed,  there  are  no  experiments  which  directly  appl}^  to  the 
cases  usually  met  with.  The  following  table  is  drawn  up  from 
experiments  made  in  a  confined  channel,  the  bottom  and  sides 
of  the  channel  being  formed  of  rough  boards : — 


282 


CIVIL  ENGINEERING. 


Stages  of  accumu- 
lation termed 

Velocity  of 
river  in  feet 
per  second. 

Nature  of  the  bottom  which  just  bears 
such  velocities. 

Specific  gravity 
of  the  mate- 
rial. 

Ordinary  floods.... 
Uniform  tenors  

J  3.2 
1  2.17 
(  1.07 
-^0.62 
0.71 
0.351 
0.26 

Angular  stones,  the  size  of  a  hen's  egg. . 
Rounded  pebbles  one  inch  in  diameter, 
•ravel  of  the  size  of  garden  beans  

2.25 

2.614 

2.545 

2.545 

2.36 

2.545 

2.64 

Sand,  the  grains  the  size  of  aniseeds . . . 

Dull  

562.  Bays.  As  a  general  rule,  there  should  be  an  odd 
number  of  bays,  whenever  the  width  of  the  water-way  is  too 
great  to  be  spanned  by  a  single  arch.  Local  circumstances 
may  require  a  departure  from  this  rule ;  but  when  departed 
from,  it  will  be  at  the  cost  of  architectural  effect ;  since  no 
secondary  feature  can  occupy  the  central  point  in  any  archi- 
tectural composition  without  impairing  the  beauty  of  the 
structure  to  the  eye ;  and  as  the  arches  are  the  main  features 
of  a  stone  bridge,  the  central  point  ought  to  be  occupied  by 
one  of  them. 

The  width  of  the  bays  will  depend  mainly  upon  the  char- 
acter of  the  current,  the  nature  of  the  soil  upon  which  the 
foundations  rest,  and  the  kind  of  material  that  can  be  obtained 
for  the  masonry. 

For  streams  with  a  gentle  current,  which  are  not  subject  to 
heavy  freshets,  narrow  bays,  or  those  of  a  medium  size  may 
be  adopted,  because,  even  a  considerable  diminution  of  the 
natural  water-way  will  not  greatly  affect  the  velocity  under 
the  bridge,  and  the  foundations  therefore  will  not  be  liable  to 
be  undermined.  The  difficulty,  moreover,  of  laying  the  foun- 
dations in  streams  of  this  character  is  generally  inconsiderable. 
For  streams  with  a  rapid  current,  and  which  are,  moreover, 
subject  to  great  freshets,  wide  bays  will  be  most  suitable,  in 
order,  by  procuring  a  wide  water-way,  to  diminish  the  danger 
to  the  points  of  support,  in  placing  as  few  in  the  stream  as 
practicable. 

563.  Classification  of  Arches.  Arches  are  classed,  ac- 
cording to  their  concave  surface,  as:  cylindrical,  conical, 
conoidal,  warjped,  annular,  groined,  cloistered,  and  domes. 

A  right  arch  is  one  in  which  the  axis  is  perpendicular  to 
the  face  ;  and  an  oblique  arch  is  one  in  which  the  axis  is  not 
perpendicular  to  the  face. 

A  rampant  arch  is  one  in  which  the  axis  is  not  in  a  horizon- 
tal plane. 

564.  Surfaces  of  the  Arch.  The  soffit  is  the  inner  con- 
cave surface. 


AKCHES. 


283 


The  lack  is  the  external  surface. 

The /ace  of  the  arch  is  the  end  surface. 

565.  Lines  of  the  Arch.  The  springing  lines  are  the  in- 
tersections of  the  soffit  with  the  abutment ;  as  a',  c',  Fig.  121. 
In  Fig.  115,  B  is  the  projection  of  a  springing  line. 

The  span  is  the  chord  of  the  curve  of  right  section,  as 
DB,  Fig.  115. 


Fig.  115 — Represents  an  oval  curve  of 
three  centres,  the  arcs  of  which  are 
each  60°. 

DB,  span  of  the  curve. 

AC,  rise. 

P,  O,  and  R,  centres  of  the  arcs  of  60°. 
DOB  is  the  intrados. 


The  axis  of  the  arch  is  the  line  passing  through  the  centres 
of  the  span. 

The  rise  is  the  versed  sine  of  the  curve  of  right  section,  as 
AC,  Fi^.  115. 

The  intrados  is  the  intersection  of  the  soffit  with  the  face 
of  the  arch,  as  DCB. 

The  extrados  is  the  intersection  of  the  back  of  the  arch 
with  the  face. 

The  intrados  may  be  defined  as  the  inner  curve  of  a  verti- 
cal right  section,  and  the  extrados  as  the  outer  one. 

The  crown  is  the  highest  line  of  the  soffit. 

The  coursing  joints  iare  those  lines  which  run  lengthwise  of 
the  arch,  and  separate  the  several  courses  of  the  stones. 

The  heading  or  ring  joints  are  those  lines  which  separate 
the  stones,  and  are  nearly  or  quite  parallel  to  the  face  of  the 
arch. 

566.  Volumes  of  the  Arch.  The  blocks  of  stone  which 
form  the  body  of  the  arch  are  called  voussoirs. 

The  keystone  is  the  highest  stone  of  the  arch. 

The  impost  stones  are  the  highest  stones  of  the  abutment, 
and  upon  wliich  the  arch  directly  rests. 


284 


CmL  ENGINEERING. 


567.  Cylindrical  Arch.  This  is  tlie  most  usual  and 
the  simplest  form  of  arch.  The  soffit  consists  of  a  portion 
of  a  cylindrical  surface.  When  the  section  of  the  cylin- 
der perpendicular  to  the  axis  of  the  arch,  termed  a  right 
section^  cuts  from  the  surface  a  semicircle,  the  arch  is  termed 
a  full  centre  arch ;  when  the  section  is  an  arc  less  than  a 
semicircle,  it  is  termed  a  segmental  arch  /  when  the  section 
gives  a  semi-ellipse,  it  is  termed  an  ellijptical  arch  ;  when  the 
section  gives  a  curve  resembling  a  semi-ellipse,  formed  of  arcs 
of  circles  tangent  to  each  other,  the  arch  is  termed  an  oval^ 
(Fig.  115,  or  oasket  handle),  and  is  called  a  curve  of  three^ 


five,  seven,  etc.,  centres.  In  order  to  make  the  curve  horizon- 
tal at  the  crown  and  symmetrical  in  reference  to  a  vertical 
line  through  the  centre,  there  must  be  an  odd  number  of  arcs. 
When  the  intrados  is  composed  of  two  arcs  meeting  at  the 
highest  point  of  the  curve,  it  is  called  a  pointed,  (Fig.  116,) 
or  an  obtuse  or  swhased  arch,  (Fig.  117.) 


ARCHES. 


285 


568.  Oblique  Arches.  If  the  obliquity  of  the  arch  is 
small,  it  may  be  constructed  like  the  right  arch,  but  when  the 
obliquity  is  considerable,  or  in  other  words  when  the  angle 
between  the  axis  and  face  is  considerably  less  or  greater  than 
90  degrees,  the  pressure  upon  the  voussoirs  near  the  end  of  the 
springing  lines  would  be  very  oblique  to  the  beds,  and  at  the 
acute  angles  would  tend  to  force  the  voussoirs  out  of  place  if 
the  coursing  joints  are  made  parallel  to  the  axis.  To  obviate 
this  defect  the  coursing  joints  are  inclined  to  the  cylindrical 
elements,  as  will  now  be  explained. 

An  ideal  mode  of  determining  the  coursing  joints  is  to 
conceive  the  arch  to  be  intersected  by  an  indefinite  number 
of  vertical  planes  parallel  to  the  face,  thus  making  an  indefi- 
nite number  of  curves  like  the  end  ones.  Then  begin  at  any 
point,  as  Fig.  118,  and  pass  a  line  along  the  sofiit  so  as  to 
cut  all  the  former  curves  at  right  angles,  and  we  have  an 
ideal  coursing  joint.  The  line  d  Fig.  118,  represents  such 
a  line.  Other  similar  curves  are  also  shown.  The  equation 
of  these  when  developed  is  logarithmic.  They  are  all  asymp- 
totes to  the  springing  line.  The  plan  of  these  curves  is  shown 
in  Fig.  119.  A  suitable  number  of  vertical  intersections  may 
be  selected  for  determining  the  ring-joints,  portions  of  which 
only  are  used,  as  h  a,  Fig.  118,  and  h\  a',  Fig.  119. 


Fig.  118. 


A 

'  c 

i  1 

1  ) 

i  1 

1 

B 

Fig.  118— Elevation  of  an  oblique 
arch,  in  which  the  coursing  joints 
d  c,  etc.,  are  normal  to  the  ring- 
joints,  6  a,  etc. 

B  is  the  abutment. 

A  the  filling  over  the  back. 

Fig.  119— Plan  of  the  oblique  arch 
shown  in  Fig.  118,  showing  the  plan 
of  the  coursing  joint  and  heading 
joints. 


Fig.  119. 


286 


CIVIL  ENGINEERING. 


This  mode  of  determining  the  coursing  joints  is  very  ob- 
jectionable in  practice,  because  the  voussoirs  must  constantly 
vary  in  width  as  we  pass  from  one  end  to  the  other  ;  and  as 
the  bed-surfaces  are  warped,  it  makes  it  exceedingly  difficult 
to  make  the  voussoirs  of  proper  shape. 

The  method  of  making  the  coursing  joints  nearly  or  quite 
parallel  to  each  other,  sometimes  called  the  English  method, 
is  more  simple,  and  gives  as  good  results  as  the  preceding 
method. 


Fig.  120. 


Fig.  120  is  the 
plan  of  an  oblique 
arch. 
Tc  I  is  the  axis,  a 
c  the  ppnnging 
line,  a  k  the  face; 
a  b  and  c  A  the  de- 
velopment of  the 
intrados  of 
oblique  section. 
The  right  sec- 
tion, m/,  is  the 
arc  of  a  circle;  h 
f  and  i  g  are  hor- 
izontal projec- 
tions of  heading 
joints;  /  n  is  the 
development  of 
the  joint  h  f.  g 
2,  c  3,  etc.,  are 
the  developments 
of  coursing 
joints. 
Fig.  121  is  tho 
elevation  of  an 
oblique  arch,  of 
which  Fig.  120  is 
the  plan, 
o  c'  o  is  the  sof- 
fit. 

a  c'  is  the  spring- 
ing line. 
d  o,  spiral  cours- 
ing joint. 
C  is  a  point  di- 
rectly below  the 
axis,  from  which 
all  the  joints,  as 
p  o,  in  the  face 
radiate. 


Fig.  121. 


Fig.  121  is  the  elevation  of  such  an  oblique  arch,  and  Fig. 
120  is  the  plan.    The  system  here  shown  is  sometimes  called 

Buck's  System."  In  order  to  construct  this  system 
graphically,  we  conc^eive  that  the  soffit  is  developed,  or 
rolled  out  about  the  springing  line  a  c.    Let  mj^  be  a  right 


STONE  BRIDGES. 


287 


section  (which  is  here  supposed  to  be  circular).  Conceive 
that  it  is  revolv^ed  down  to  coincide  with  the  horizontal  plane, 
and  that  the  circumference  is  divided  into  a  convenient  num- 
ber of  equal  parts,  and  through  the  points  of  division  conceive 
that  cylindrical  elements  are  drawn,  as  shown  in  the  plan. 
In  the  development  the  circumference  of  the  semicircle  will 
become  the  liney^,  and  the  cylindrical  elements  will  be,  as 
shown,  parallel  to  the  springing  line  ac.  From  the  points 
where  the  horizontal  projections  of  the  cylindri(;al  elements 
intersect  the  face  ah^  draw  lines  parallel  to  fh,  and  note  their 
intersections  with  the  developed  position  of  the  cylindrical 
elements,  and  the  curve  adh  through  these  points  will  be  the 
development  of  the  intrados  of  oblique  section.  In  a  similar 
way  find  c  A. 

join  a  h  with  a  straight  line,  and  divide  it  into  as  many 
equal  parts  as  there  are  to  be  voussoirs  'in  the  face.  In  the 
figure  there  are  eight  such  parts.  When  there  is  an  even 
number  there  will  be  a  joint  at  the  crown,  but  when  an  odd 
number  there  will  be  the  appearance  of  a  keystone  at  the 
crown.  From  c  at  the  end  of  the  springing-line  a  g 
draw  a  perpendicular  cd  to  the  line  a  5,  and  if  it  passes 
through  one  of  the  divisions  previously  determined  on  a  h,  we 
proceed  with  the  construction ;  but  if  it  does  not,  we  make 
such  a  change  in  the  data  as  will  make  it  perpendicular. 
This  may  be  done  in  several  ways.  We  may  erect  a  perpen- 
dicular to  a  h  from  the  joint  which  is  nearest  the  foot  of  the 
perpendicular  previously  drawn,  and  note  where  it  inter- 
sects the  springing-line,  and  change  the  length  of  the  arch  so 
that  it  will  pass  through  that  point.  Or  we  may  change  the 
obliquity  of  the  arch,  or  change  the  number  of  divisions  of 
the  line  a  h.  If  the  foot  of  the  perpendicular  should  fall  near 
a  division,  the  line  may  be  changed  so  as  to  pass  through  the 
point  and  leave  it  slightly  out  of  a  perpendicular.  We  might 
also  disregard  the  condition  that  the  perpendicular  6?  (3  should 
pass  through  the  end  of  the  springing-line  a  c;  but  this  is  ob- 
jectionable, because  the  opposite  sides  of  the  arch  would  then 
not  be  alike. 

Having  fixed  the  position  of  c  d,  we  proceed  to  draw  lines 
through  the  several  points  of  division  of  a  5,  parallel  to  g  d.  It 
should  be  observed  that  points  through  which  these  parallel 
lines  are  drawn  are  on  the  straight  line  a  dh^  and  not  on  the 
curved  line  a  1,  2,  etc.  The  parallel  lines  thus  drawn  are  the 
coursing  joints.  The  development  of  the  ring  joints  fn,  etc., 
are  per^Dendicular  to  the  developed  coursing  joints,  and  hence 
will  be  normal  to  each  other  in  their  true  position  in  the 


288 


CmL  ENGINEERINQ. 


arch;  and  hence  it  is  evident  that  the  intrados  in  oblique 
section  a  h  will  not  be  perpendicular  to  the  coursing  joints. 
And  since  the  projection  of  the  face  is  a  straight  line,  ah^  it 
is  evident  that  the  horizontal  projection  of  a  ring  joint  will  be 
a  curved  line^/A,  the  position  or  which  may  be  determined 
by  reversing  the  process  by  which  1  2  J  was  found.  The 
horizontal  projection  of  the  coursing  joints  will  also  be  curved 
lines. 

This  construction  evidently  makes  the  divisions  a  1-12-23, 
etc.,  on  the  curved  line  adh^  unequal.  The  space  a  1  on 
the  development  is  laid  off  on  the  arc  in  the  elevation  from  a. 
The  space  1-2  is  next  laid  off,  and  so  on.  By  developing  the 
extrados  and  determining  the  points  of  division  on  the  back 
of  the  arch,  we  may  construct  the  radial  lines  in  the  face  of 
the  arch.  These  lines  are  slightly  curved  in  the  arch,  but  it 
is  found,  by  constructing  the  arch  on  a  large  scale,  that  the 
chords  of  the  arcs  o  J9,  etc.,  all  pass  through  a  common  point 
C.  The  coursing  joints  and  ring  joints  in  the  elevation  are 
easily  determined  from  the  plan. 

The  bed-surfaces  of  the  voussoirs  may  be  generated  by  con- 
ceiving a  radial  line  to  pass  through  one  corner  of  them 
(which  will  be  normal  to  the  soffit)  and  moved  along  on  a 
coursing  joint,  keeping  it  constantly  normal  to  the  soffit. 
This  line  will  generate  a  true  helicoidal  surface.  The  end 
surfaces  of  the  voussoirs  are  generated  in  a  similar  way  by 
moving  a  radial  line  along  a  ring  joint,  and  hence  these  sur- 
faces are  also  helicoidal.  The  lengths  of  the  end  voussoirs, 
measured  on  the  back  of  the  arch  next  to  the  oblique  angles, 
will  be  shorter  than  those  next  to  the  acute  angles,  w^hile  all 
those  in  the  body  of  the  arch  will  be  like  each  other. 

Mr.  Hart,  an  English  author,  proposed  a  method  which 
differed  from  the  one  above  explained  in  the  following  par- 
ticulars :  The  spaces  in  the  curved  line  adh  were  made  equal 
to  each  other ;  the  coursing  joints  were  straight,  and  passed 
through  the  points  of  division  at  the  opposite  ends  of  the  arch 
in  the  developed  intrados  ;  hence,  the  coursing  joints  in  this 
system  are  not  parallel  to  each  other.  Another  distinction  is, 
the  ring  joints  and  end-faces  of  all  the  voussoirs  are  parallel 
to  the  end  of  the  arch,  and  hence  the  end-faces  are  plane. 
This  might  simplify  the  construction,  but  it  does  not  use  the 
material  from  which  the  voussoirs  are  cut  as  economically  as 
the  preceding  system.  In  this  system  the  bed-surfaces  are 
helicoidal,  as  in  the  preceding  system.  The  preceding  system 
seems  to  be  thoroughly  scientific  and  quite  as  easily  executed 
as  the  latter,  or  of  any  other  conceivable  system  in  which  the 


STONE  BRIDGES. 


289 


joints  are  spiral.  In  practice,  temjplets  and  'bevels  are  made,  in 
order  to  guide  the  workmen  in  making  the  angles  and  surfaces 
of  the  voussoirs. 

569.  Arched  Bridges.  Cylindrical  arches  with  any  of  the 
usual  forms  of  curve  of  intrados  may  be  used  for  bridges. 
The  selection  will  be  restricted  by  the  width  of  the  bay,  the 
highest  water-level  during  freshets,  the  approaches  to  the 
bridge,  and  the  architectural  effect  which  may  be  produced 
by  the  structure,  as  it  is  more  or  less  exposed  to  view  at  the 
intermediate  stages  between  high  and  low  water. 

Oval  and  segment  arches  are  mostly  preferred  to  the  full 
centre  arch,  particularly  for  medium  and  wide  bays,  for  the 
reasons  that,  for  the  same  level  of  roadway,  they  affoi'd  a  more 
ample  water-way  under  them,  and  their  heads  and  spandrels 
offer  a  smaller  surface  to  the  pressure  of  the  water  during 
freshets  than  the  full  centre  arch  under  like  circumstances. 

The  level  of  the  springing  lines  will  depend  upon  the  rise 
of  the  arches,  and  the  height  of  their  crowns  above  the  water- 
level  of  the  highest  freshets.  The  crown  of  the  arches  should 
not,  as  a  general  rule,  be  less  than  three  feet  above  the  high- 
est known  water-level,  in  order  that  a  passage-way  may  be 
left  for  floating  bodies  descending  during  freshets.  Between 
this,  the  lowest  position  of  the  crown,  and  any  other,  the  rise 
should  be  so  chosen  that  the  approaches,  on  the  one  hand, 
may  not  be  unnecessarily  raised,  nor,  on  the  other,  the  spring- 
ing lines  be  placed  so  low  as  to  mar  the  architectural  effect 
of  the  structure  during  the  ordinary  stages  of  the  water. 

When  the  arches  are  of  the  same  size,  the  axis  of  the  road- 
way and  the  principal  architectural  lines  which  run  lengthwise 
along  the  heads  of  the  bridge,  as  the  top  of  the  parapet,  the 
cornice,  etc.,  etc.,  will  be  horizontal,  and  the  bridge,  to  use  a 
common  expression,  be  on  a  dead  level  throughout.  This  has 
for  some  time  been  a  favorite  feature  in  bridge  architecture, 
few  of  the  more  recent  and  celebrated  bridges  being  without 
it,  as  it  is  thought  to  give  a  character  of  lightness  and  bold- 
ness to  the  structure. 

570.  Centres.  Before  an  arch  is  constructed  a  strong  sup- 
port or  framework  is  erected  to  sujDport  the  arch  until  the 
work  is  completed.  This  support  is  called  the  centering  of 
the  arch.  It  must  be  made  strong,  and  so  as  to  settle  as  little 
as  possible  while  the  masonry  is  being  erected  ;  and  in  arches 
of  long  span  it  must  be  so  erected  and  supported  that  it 
may  be  removed  without  causing  local  or  cross  strains  in 
the  arch.  To  accomplish  this,  the  centering  should  be  re- 
moved from  the  entire  soffit  at  the  same  time.    It  is  espe- 


290 


•CIVIL  •ENGINEERING. 


cially  detrimental  to  relieve  one  side  whilst  the  other  side  is 
firmly  supported. 

571.  Means  used  for  striking  Centres.  When  the  arch  is 
completed  the  centres  are  detached  from  it,  or  struck.  To 
effect  this  in  hii-ge  centres  an  arrangement  of  wedge  blocks 
is  used,  termed  the  striking -plates^  by  means  of  which  the 
centre  may  be  gradually  lowered  and  separated  from  the 
soffit  of  the  arch.  This  arrangement  consists  (Fig.  125)  in 
forming  steps  upon  the  upper  surface  of  the  beam  which 
forms  the  framed  support  to  receive  a  wedge-shaped  block, 
on  w^hich  another  beam,  having  its  under  surface  also  ar- 
ranged with  steps,  rests.  The  struts  of  the  rib  either  abut 
against  the  upper  surface  of  tlie  top  beam,  or  else  are  inserted 
into  cast-iron  sockets,  termed  shoe-plates^  fastened  to  this 
surface.  The  centre  is  struck  by  driving  back  the  wedge 
block. 

572.  When  the  struts  rest  upon  intermediate  supports  be- 
tween the  abutments,  double  or  folding  wedges  may  be 
placed  under  the  struts,  or  else  upon  the  back  pieces  or  the 
ribs  under  each  bolster.  The  latter  arrangement  presents 
the  advantage  of  allowing  any  part  of  the  centre  to  be  eased 
from  the  solht,  instead  of  detaching  the  whole  at  once  as  in 
the  other  methods  of  striking  wedges.  This  method  was 
employed  for  the  centres  of  Grosvenor  Bridge  (Fig.  124), 
over  the  river  Dee  at  Chester,  and  was  perfectly  successful 
both  in  allowing  a  gradual  settling  of  the  arch  at  various 
points,  and  in  the  operation  of  striking. 

573.  A  novel  application  of  sand  to  the  striking  of  centres 
has  lately  been  made  with  success.  Vessels  containing  the 
sand  are  placed  on  the  supports  for  the  centres,  and  are  so 
arranged  near  the  bottom  that  the  sand  can  be  allowed  to  run 
out  slowly  when  the  time  comes  for  striking.  The  centres 
are  placed  on  these  vessels  and  keyed  up  in  the  usual  way. 
To  lower  them,  the  sand  is  allowed  to  run  out  and  let  the 
centres  gradually  down.  This  method  has  the  advantaf^e  of 
steadiness  of  lowering  each  rib  of  the  centre,  and  of  not 
allowing  one  to  come  down  more  rapidly  than  the  others. 
After  the  sand  has  all  run  out,  the  centres  can  be  taken  down 
in  the  ordinary  manner. 

574.  For  small  light  arches  (Fig.  122)  the  ribs  may  be 
formed  of  two  or  more  thicknesses  of  short  boards,  hrmly 
nailed  together ;  the  boards  in  each  course  abutting  end  to 
end  by  a'  joint  in  the  direction  of  the  radius  of  curvature  of 
the  arch,  and  breaking  joints  with  those  of  the  other  course. 
The  ribs  are  shaped  to  the  form  of  the  intrados  of  the  arch, 


STONE  BRIDGES. 


291 


to  receive  the  bolsters,  which  are  of  battens  cut  to  suitable 
lengths  and  nailed  to  the  ribs. 


575.  For  heavy  arches  with  wide  spans,  when  firm  inter- 
mediate points  of  support  can  be  procured  between  the  abut- 
ments, the  back  pieces  (Fig.  123)  may  be  supported  by  shores 


Fig.  123 — Represents  the 
rib  of  a  centre  with  in- 
termediate points  of 
support, 
a,  back  pieces  of  the  rib 
which  receive  the  bol- 
sters /. 
6,  6,  struts  which  support 

the  back  pieces. 
«,  c,  braces. 

c,  solid  beam  resting  on 
the  intermediate  sup- 
ports d,  d,  which  re- 
ceive the  ends  of  the 
struts  6  b. 

placed  under  the  blocks  in  the  direction  of  the  radii  of  curva- 
ture of  the  arch,  or  of  inclined  struts  (Fig.  124)  resting  on  the 
points  of  support.  The  shores,  or  struts,  are  prevented  from 
bending  by  l3races  suitably  placed  for  the  purpose. 

If  intermediate  points  of  support  cannot  be  obtained,  a 
broad  framed  support  must  be  made  at  each  abutment  to 
receive  the  extremities  of  the  struts  that  sustain  the  back 
pieces.  The  framed  support  (Fig.  125)  consists  of  a  heavy 
beam  laid  either  horizontally  or  inclined,  and  is  placed  at  that 
joint  of  the  arch  (the  one  which  makes  an  angle  of  about 
30°  with  the  horizon)  where  the  voussoirs,  if  unsupported 
beneath,  would  slide  on  their  beds.  This  beam  is  borne  by 
shores,  which  find  firm  points  of  support  on  the  foundations 
of  the  abutment. 

The  back  pieces  of  the  centre  (Fig.  125)  may  be  supported 
by  inclined  struts,  which  rest  immediately  upon  the  framed 
support,  one  of  the  two  struts  under  each  block  resting  upon 
one  of  the  framed  supports,  the  other  on  the  one  on  the  oppo- 


292 


CIVIL  ENGINEEKING. 


Fig.  124 — Represents  a  part  of  the  rib  of  Grosvenor  Bridge  over  the  Dee  at  Chester.    Span  200 
feet. 

A,  A,  intermediate  points  of  support. 

a,  a,  a,  struts  resting  upon  cast-iron  sockets  on  the  supports  A. 

6,  6,  two  courses  of  plank  each  4^  inches  thick  bent  over  the  struts  a,  a,  to  the  form  of  the 

arch,  the  courses  breaking  joints, 
c,  c,  folding  wedges  laid  upon  the  back  pieces  6  of  each  rib  to  receive  the  bolsters  on  which 

the  voussoirs  are  laid. 

site  side,  the  two  struts  being  so  placed  as  to  make  equal 
angles  with  the  radius  of  curvature  of  the  arch  drawn  through 
the  middle  point  of  the  block.  Bridle  pieces,  placed  in  the 
direction  of  the  radius  of  curvature,  embrace  the  blocks  and 
struts  in  the  usual  manner,  and  prevent  tlie  latter  from  sag- 
ging. This  combination  presents  a  figure  of  invariable  form, 
as  the  strain  at  any  one  point  is  received  by  the  struts  and 
transmitted  directly  to  the  fixed  points  of  support.  It  has 
the  disadvantage  of  requiring  beams  of  great  length  when  the 
span  of  the  arch  is  considerable,  and  of  presenting  frequent 
crossing  of  the  struts  where  notches  will  be  requisite,  and  the 
strength  of  the  beams  thereby  diminished. 

The  centre  of  Waterloo  Bridge,  over  the  Thames  (Fig.  125), 
was  framed  on  this  principle.  To  avoid  the  inconveniences 
resulting  from  the  crossing  of  the  struts,  and  of  building 
beams  of  sufficient  length  where  the  struts  could  not  be  pro- 
cured from  a  single  beam,  the  device  was  adopted  of  receiv- 
ing the  ends  of  several  struts  at  the  points  of  crossing  into 
a  large  cast-iron  socket  suspended  by  a  bridle  piece.' 

576.  When  the  preceding  combination  cannot  be  employed, 
a  strong  truss  (Fig.  126),  consisting  of  two  inclined  struts, 
resting  upon  the  framed  supports,  and  abutting  at  top  against 
a  straining  beam,  may  be  formed  to  receive  the  ends  of  some 


CENTEKS  FOR  ARCHES. 


293 


Pig.  125— Represents  a  part  of  a  rib  of  Waterloo  Bridge  over  the  Thames. 

a,  a,  b,  three  heavy  beams,  forming  the  striking  plate.%  which  with  the  shores  A,  A,  form  the^ 
framed  support  for  the  struts  of  the  centre.  4 

c,  c,  struts  abutting  against  the  blocks  g,  g  placed  under  the  joints  of  the  back  pieces/,  /. 

d,  d,  bridle  or  radial  pieces  in  pairs  which  are  confined  at  top  and  bottom  between  the  hori^ 

zontal  ties  n,  n  of  the  ribs,  also  in  pairs. 
«,  <J,  cast-iron  sockets. 

m,  m,  bolsters  of  the  centre  resting  on  the  back  pieces  f. 


Fig.  126 — Represents  a  frame 
for  a  rib  in  which  the  two 
inclined  struts  ft,  b  and  the 
straining  beams  c  form  in- 
termediate supports  for 
some  of  the  struts  that  sup- 
ort  the  back  pieces  a,  a, 

e  and  d  are  the  framed  ex- 
treme supports 


of  the  stmts  which  support  the  back  pieces.  This  combina- 
tion, and  all  of  a  like  character,  require  that  the  arch  should 
not  be  constructed  more  rapidly  on  one  side  of  the  centre 
than  on  the  other,  as  any  inequality  of  strain  on  the  two 
halves  of  the  centre  would  have  a  tendency  to  change  the 
shape  of  the  frame,  thrusting  it  in  the  direction  of  the  greater 
strain. 


CrVTL  ENGINEERING. 


577.  Style  of  Architecture.  The  design  and  construction 
of  a  bridge  should  be  governed  by  the  same  general  princi- 
ples as  any  other  architectural  composition.  As  the  object  of 
a  bridge  is  to  bear  heavy  loads,  and  to  withstand  the  effects 
of  one  of  the  most  destructive  agents  with  which  the  engineer 
has  to  contend,  the  general  character  of  its  architecture 
should  be  that  of  strength.  It  should  not  only  be  secure,  but 
to  the  apprehension  appear  so.  It  should  be  equally  removed 
from  Egyptian  massiveness  and  Corinthian  lightness ;  while, 
at  the  same  time,  it  should  conform  to  the  features  of  the 
surrounding  locality,  being  more  ornate  and  carefully  wrought 
in  its  minor  details  in  a  city,  and  near  buildings  of  a  sump- 
tuous style,  than  in  more  obscure  quarters;  and  assuming 
every  shade  of  conformity,  from  that  which  would  be  in 
keeping  with  the  humblest  hamlet  and  tamest  landscape  to 
the  boldest  features  presented  by  Nature  and  Art.  Sim- 
plicity and  strength  are  its  natural  characteristics  ;  all  orna- 
ment of  detail  being  rejected  w^hich  is  not  of  obvious  utility, 
and  suitable  to  the  point  of  view  from  which  it  must  be  seen  ; 
as  well  as  all  attempts  at  boldness  of  general  design  which 
might  give  rise  to  a  feeling  of  insecurity,  however  unfounded 
in  reality.  The  heads  of  the  bridge,  the  cornice,  and  the 
parapet  should  generally  present  an  unbroken  outline ;  this, 
however,  may  be  departed  from  in  bridges  where  it  is  desira- 
ble to  place  recesses  for  seats,  so  as  not  to  interfere  with  the 
footpaths ;  in  which  case  a  plain  buttress  may  be  built  above 
each  starling  to  support  the  recess  and  its  seats,  the  utility  of 
which  will  be  obvious,  while  it  will  give  an  appearance  of 
additional  strength  when  the  height  oi  the  parapet  above  the 
starlings  is  at  all  considerable. 

578.  Construction.  The  methods  of  laying  the  founda- 
tions of  structures  of  stone,  c%c.,  described  under  the  article 
of  Masonry,  are  alike  applicable  to  all  structures  which  come 
under  this  denomination. 

579.  Various  expedients  have  been  tried  to  secure  the  bed 
of  the  natural  water-way  around  and  between  the  piers ; 
among  the  most  simple  and  efficacious  of  which  is  that  of 
covering  the  surface  to  be  protected  by  a  bed  of  stone  broken 
into  fragments  of  sufficient  bulk  to  resist  the  velocity  of  the 
current  in  the  bays,  if  the  soil  is  of  an  ordinary  clayey  mud ; 
but,  if  it  be  of  loose  sand  or  gravel,  the  surface  should  be 
first  covered  by  a  bed  of  tenacious  clay  before  the  stone  be 
thrown  in.  The  voids  between  the  blocks  of  stone,  in  time, 
become  filled  with  a  deposit  of  mud,  which,  acting  as  a 
cement,  gives  to  the  mass  a  character  of  great  durability. 


STONE  BRIDGES. 


295 


580.  The  foundation  courses  of  the  piers  should  be  formed 
of  heavy  blocks  of  cut  stone  bonded  in  the  most  careful 
manner,  and  carried  up  in  offsets.  The  faces  of  the  piers 
should  be  of  cut  stone  well  bonded.  They  may  be  built 
either  vertically,  or  with  a  slight  batter.  Their  thickness  at 
the  impost  should  be  greater  than  what  would  be  deemed 
sufficient  under  ordinary  circumstances ;  as  they  are  exposed 
to  the  destructive  action  of  the  current,  and  of  shocks  from 
heavy  floating  bodies ;  and  from  the  loss  of  weight  of  the 
parts  immersed,  owing  to  the  buoyant  effort  of  the  water, 
their  resistance  is  decreased.  The  most  successful  bridge 
architects  have  adopted  the  practice  of  making  the  thickness 
of  the  piers  at  the  impost  between  one  sixth  and  one  eighth 
of  the  span  of  the  arch.  The  thickness  of  the  piers  of  the 
bridge  of  JSTeuilly,  near  Paris,  built  by  the  celebrated  Perronet, 
whose  works  form  an  epoch  in  modern  bridge  architecture,  is 
only  one  ninth  of  the  span,  its  arches  also  being  remarkable 
for  the  boldness  of  their  curve. 

581.  The  usual  practice  is  to  give  to  all  the  piers  the  same 
proportional  thickness.  It  has,  however,  been  recommended 
by  some  engineers  to  give  sufficient  thickness  to  a  few  of  the 
piers  to  resist  the  horizontal  thrust  of  the  arches  on  either  side 
of  them,  and  thus  secure  a  part  of  the  structure  from  ruin, 
should  an  accident  happen  to  any  of  the  other  piers.  These 
masses,  to  which  the  name  abutment  jpiers  has  been  applied, 
would  be  objectionable  from  the  diminution  of  the  natural 
water-way  that  w^ould  be  caused  by  their  bulk,  and  from  the 
additional  cost  for  their  construction,  besides  impairing  the 
architectural  effect  of  the  structure.  They  prese'nt  the 
advantage,  in  addition  to  their  main  object,  of  permitting  the 
bridge  to  be  constructed  by  sections,  and  thus  procure  an 
economy  in  the  cost  of  the  wooden  centres  for  the  arches. 

582.  The  projection  of  the  starlings  beyond  the  heads  of 
the  bridge,  their  form,  and  the  height  given  to  them  above 
the  springing  lines,  will  depend  upon  local  circumstances. 
As  the  main  objects  of  the  starlings  are  to  form  a  fender  or 
guard  to  secure  the  masonry  of  the  spandrels,  &c.,  from 
being  damaged  by  floating  bodies,  and  to  serve  as  a  cut-water 
to  turn  the  current  aside,  and  prevent  the  formation  of  whirls, 
and  their  action  on  the  bed  around  the  foundations,  the  form 
given  to  them  should  subserve  both  these  purposes.  Of  the 
different  forms  of  horizontal  section  which  have  been  given 
to  starlings  (Figs.  127,  128,  129,  130),  the  semi-ellipse,  from 
experiments  carefully  made,  with  these  ends  in  view,  appears 
best  to  satisfy  both  objects. 


296 


CIVIL  ENGINEEEING. 


Fig.  127. 


Tigs.  127,  128,  and  129— Repre^ 
sent  horizontal  se<'tions  of 
starlings  A  of  the  more  usual 
forms,  and  part  of  the  pier  B 
above  the  foundation  courses. 
Fig.  130  represents  the  plan  of 
the  hood  of  a  starling  laid  in 
courses,  the  general  shape  be- 
ing that  of  the  quarter  of  a 
sphere. 


The  np  and  down  stream  starlings,  in  tidal  rivers  not  sub- 
ject to  freshets  and  ice,  usually  receive  the  same  projections, 
which,  when  their  plan  is  a  semi-ellipse,  must  be  somewhat 
greater  than  the  semi-width  of  the  pier.  Their  general  verti- 
cal outline  is  columnar,  being  either  straight  or  swelled  (Figs. 
131,  132,  133,  134).    They  should  be  built  as  high  as  the  ordi- 


Tig.  131— Represents  in  elevation  starlings  A,  their  hoods  B,  the  vonssoirs  C.  the  spandrels 
D,  and  the  combination  of  their  courses  and  joints  with  each  other  in  an  oval  arch  of  three 
centres. 

E,  parapet ;  F,  cornice. 

nary  highest  water-level.  They  are  finished  at  top  with  a  cop- 
ing stone  to  preserve  the  masonry  from  the  action  of  rain, 
&c. :  this  stone,  termed  tlie  hood,  may  receive  a  conical,  a 
spheroidal,  or  any  other  shape  which  will  subserve  the  object 
in  view,  and  produce  a  pleasing  arcliitectui'al  effect,  in  keep- 
ing with  the  locality. 


STONE  BRIDGES. 


297 


,  Fig.  132— Represents  in  elevation  the  combinations  of  the  same  elements  as  in  Pig.  131  for  a 

fiat  segmental  arch. 


Fig.  133- Kepresents  in  elevation  the  combinations  of  the  same  elements  as  in  Fig.  132,  from 

the  bridge  of  Neuilly,  and  oval  of  eleven  centres. 
om,  curve  of  intrados. 

on,  arc  of  circle  traced  on  the  head  of  the  bridge. 


Fig.  1S4 — Represents  a  cross  section 
and  elevation  through  the  crown 
of  Fig.  132,  showing  the  ar- 
rangement also  of  the  roadway, 
footpaths,  parapet,  and  cornice. 


298 


CIVIL  ENGINEEEING. 


In  streams  subject  to  freshets  and  ice,  the  up-stream  star- 
lings should  receive  a  greater  projection  than  those  down 
stream,  and,  moreover,  be  built  in  the  form  of  an  inclined 
plane  (Fig.  135)  to  facilitate  the  breaking  of  the  ice,  and  its 
passage  through  the  arches. 


Fig.  135— Represents  a  side  elevation 
and  plan  N  of  a  pier  of  the  Poto- 
mac aqueduct,  arranged  with  an 
ice-breaker  starling. 

A,  up-stream  starling,  with  the  in- 
clined ice-breaker  D,  which  riseg 
from  the  low-water  level  above 
that  of  the  highest  freshets. 

B,  down-stream  starling. 

C,  face  of  pier. 

E,  top  of  pier. 

F,  horizontal  projection  of  top  of 
ice  breaker. 

GG,  horizontal  projection  of  faces 
of  pier  and  starlings. 


583.  Where  the  banks  of  a  water-course  spanned  by  a 
bridge  are  so  steep  and  difficult  of  access  that  the  roadway 
must  be  raised  to  the  same  level  with  their  crests,  security 
for  the  foundation,  and  economy  in  the  construction  demand 
that  hollow  or  ojpen  jpiers  be  used  instead  of  a  solid  mass  of 
masonry.  A  construction  of  this  kind  requires  great  pre- 
caution. The  facing  courses  of  the  piers  must  be  of  heavy 
blocks  dressed  \vith  extreme  accuracy.  The  starlings  must 
be  built  solid.  The  faces  must  be  connected  by  one  or  more 
cross  tie-walls  of  heavy,  well-bonded  blocks  ;  the  tie-walls  be- 
ing connected  from  distance  to  distance  vertically  by  strong 
tie-blocks  ;  or,  if  the  width  of  the  pier  be  considerable,  by  a 
tie-wall  along  its  centre  line. 

584.  The  foundations,  the  dimensions,  and  the  form  of  the 
abutments  of  a  bridi2:e  will  be  regulated  upon  the  same  princi- 
ples as  the  like  parts  of  other  arched  structures;  a  judicious 
conformity  to  the  character  of  strength  demanded  by  the 


STONE  BRIDGES. 


299 


structure,  and  to  the  requirements  of  the  locality,  being  ob- 
served. The  walls  which  at  the  extremities  of  the  bridge 
form  the  continuation  of  the  heads,  and  sustain  the  embank- 
ments of  the  approaches, — and  which,  from  their  widening 
out  from  the  general  line  of  the  heads,  so  as  to  form  a  gradual 
contraction  of  the  avenue  by  which  the  bridge  is  approached, 
are  termed  the  wing-walls^ — serve  as  firm  buttresses  to  the 
abutments.  In  some  cases  the  back  of  the  abutment  is  ter- 
minated by  a  cylindrical  arch  (Fig.  136)  placed  on  end,  or 
having  its  right-line  elements  vertical,  which  connects  the 


Fig.  1S6 — Eepresents  a  horizontal  section  of 
an  abutment  A,  with  curved  wing-walls B, 
B,  connected  with  a  central  buttress  C, 
and  a  cross  tie- wall  D. 


Fig.  137— Represents  a  hori- 
zontal section  of  an  abut- 
ment A,  with  straight  wing- 
walls  B,  B,  terminated  by 
return-walls  C,  C.  D,  central 
buttress. 


two  w^in^-walls.  In  others  (Fig.  137)  a  rectangular-shaped 
buttress  is  built  back  from  the  centre  line  of  the  abutment, 
and  is  connected  with  the  wing-walls  either  by  horizontal 
arches,  or  by  a  vertical  cross  tie-wall. 

585.  The  wing-walls  may  be  either  plane  surface  walls 
(Fig.  138)  arranged  to  make  a  given  angle  with  the  heads  of 
the  bridge,  or  they  may  be  curved  surface-walls  presenting 
their  concavity  (Fig.  145)  or  their  convexity  to  the  exterior  ; 
or  of  any  other  shape,  whether  presenting  a  continuous  or  a 
broken  surface,  that  the  locality  may  demand. 

586.  The  arches  of  bridges  demand  great  care  in  proper- 


300 


CIVIL  ENGINEEEING. 


tioning  the  dimensions  of  the  voussoirs,  and  procuring  accu- 
racy in  their  forms,  as  the  strength  of  the  structure,  and  the 
permanence  of  its  figure,  will  chiefly  depend  upon  the  atten- 
tion bestowed  on  these  points.  Peculiar  care  should  be  given 
in  arranging  the  masonry  above  the  piers  which  lies  between 
the  two  adjacent  arches.  In  some  of  the  more  recent  bridges, 
(Fig.  139,)  this  part  is  built  up  solid  but  a  short  distance 
above  the  imposts,  generally  not  higher  than  a  fourth  of  the 
rise,  and  is  finished  with  a  reversed  arch  to  give  greater  se- 
curity against  the  effects  of  the  pressure  thrown  upon  it. 
I  The  backs  of  the  arches  should  be  covered  with  a  water- 
tight capping  of  beton,  and  a  coating  of  asphaltum. 
I  587.  The  entire  spandrel  courses  of  the  heads  are  usually 
not  laid  until  the  arches  have  been  uncentred,  and  have  set- 
tled, in  order  that  the  joints  of  these  courses  may  not  be  sub- 
ject to  any  other  cause  of  displacement  than  what  may  arise 
from  the  effects  of  variations  of  temperature  upon  the  arches. 
The  thickness  of  the  head-walls  will  depend  upon  the 
method  adopted  for  supporting  the  roadway.  If  this  be  by  a 
filling  of  earth  between  the  head-walls,  then  their  thickness 
must  be  calculated  not  only  to  resist  the  pressure  of  the  earth 
which  they  sustain,  but  allowance  must  also  be  made  for  the 


STONE  BRIDGES. 


301 


A,  nnisn  or  soiia  spandrel  with  rever 

B,  wedge  of  etrikinp  plates. 

C,  recess  over  the  starlings  for  seats. 


effects  of  the  shocks  of  floating  bodies  in  weakening  the  bond, 
and  separating  the  blocks  from  their  mortar-bed.  The  more 
approved  methods  of  supporting  the  roadway,  except  for  very 
flat  segment  arches,  are  to  lay  the  road  materials  either  upon 
broad  flagging  stones  (Figs.  139,  140,)  which  rest  upon  thin 
brick  walls  built  parallel  to  the  head-walls,  and  supported  by 
the  piei-s  and  arches  ;  or  by  small  arches,  (Fig.  141)  for 
which  these  walls  serve  as  piers  ;  or  by  a  system  of  small 
groined  arches  supported  by  pillars  resting  upon  the  piers 


302 


CIYIL  ENGINEERmG. 


rip:.  140— Represents  a  profile  of 
Fig.  139  through  the  centre  of 
the  pier,  showing  the  arrange- 
ment of  the  roadway  and  its 
drainage,  &c. 

A,  section  of  masonry  of  pier  and 
spandrel. 

6,  6,  sections  of  walls  parallel  to 
head-wall,  which  support  the 
flagging  stone  on  which  the 
roadway  is  laid. 

c,  section  of  head-wall  and  but- 
tress above  the  starling  d. 

e,  footpath. 

/,  recess  for  seats  over  the  but- 
tress. 

o,  cornice  and  parapet. 

n,  vertical  conduit  in  the  pier 
communicating  with  two  oth- 
ers under  the  roadway  from 
the  side  channels. 


and  main  arches.  When  either  of  these  methods  is  used,  the 
head- walls  may  receive  a  mean  thickness  of  one  fifth  of  their 
height  above  the  solid  spandrel. 

588.  Superstructure.  The  superstructure  of  a  bridge  con- 
sists of  a  cornice,  the  roadway  and  footpaths,  &c.,  and  a  par- 
apet. 

The  object  of  the  cornice  is  to  shelter  the  face  of  the  head- 
walls  from  rain.  To  subserve  this  purpose,  its  projection  be- 
yond the  surface  to  be  sheltered  should  be  the  greater  as  the 
altitude  of  the  sheltered  part  is  the  more  considerable.  This 
rule  will  require  a  cornice  with  supporting  blocks,  (Fig.  142,) 
termed  modillions,  below  it,  whenever  the  projecting  part 
would  be  actually,  or  might  seem,  insecure  from  its  weight. 
The  height  of  the  cornice,  including  its  supports,  should  gen- 
erally be  equal  to  its  projections  ;  this  will  often  require  more 
or  less  of  detail  in  the  profile  of  the  cornice,  in  order  that  it 
may  not  appear  hea\^.  The  top  surface  of  the  cornice  should 
be  a  little  above  that  of  the  footpath,  or  roadway,  and  be 
slightly  sloped  outward ;  the  bottom  should  be  arranged  with 
a  suitable  larmier^  or  drip,  to  prevent  the  water  from  finding 
a  passage  along  its  under  surface  to  the  face  of  the  wall. 

589.  The  parapet  surmounts  the  cornice,  and  should  be  high 
enough  to  secure  vehicles  and  foot-passengers  from  accidents, 
without  however  intercepting  the  view  from  the  bridge.  The 
parapet  is  usually  a  plain  low  wall  of  cut  stone,  surmounted 
by  a  coping  slightly  rounded  on  its  top  surface.  In  bridges 
which  have  a  character  of  lightness,  like  those  with  flat  seg- 
ment arches,  the  parapet  may  consist  of  alternate  panels  of 


STONE  BRIDGES. 


303 


plain  wall  and  balustrades,  provided  this  arrangement  be 
otherwise  in  keeping  with  the  locality.  The  exterior  face  of 
the  parapet  should  not  project  beyond  that  of  the  heads.  The 
blocks  of  which  it  is  formed,  and  particularly  those  of  the 
coping,  should  be  firmly  secured  with  copper  or  iron  cramps. 

590.  Strong  and  durable  stone,  dressed  with  the  chisel,  or 
hammer,  should  alone  be  used  for  the  masonry  of  bridges 
where  the  span  of  the  arch  exceeds  fifty  feet.    The  interior  of 


304  CIVIL  ENGINEERING. 


Fig.  143— Represents  an  elevation  of  a  pier,  a  portion  of  two  arches,  and  the  centre  of  the 

bridge  of  which  Fig.  141  is  the  section. 

A,  face  of  starling. 

B,  hcod. 

C,  vousBoirs  with  chamfered  joints. 


the  piers,  and  the  backing  of  the  abutments  and  head-walls, 
may,  for  economy,  be  of  good  rubble,  provided  great  atten- 
tion be  bestowed  upon  the  bond  and  workmanship.  For  me- 
dium and  small  spans  a  mixed  masonry  of  dressed  stone  and 
rubble,  or  brick,  may  be  used ;  and,  in  some  cases,  brick  alone. 
In  all  these  cases  (Figs.  141,  143)  the  starlings, — the  founda- 
tion courses, — the  impost  stone, — the  ring  courses,  at  least  of 
the  heads, — and  the  key-stone,  should  be  of  good  dressed  stone. 
The  remainder  may  be  of  coursed  rubble,  or  of  the  best  brick, 


STONE  BRIDGES. 


305 


for  the  facing,  with  good  rubble  or  brick  for  the  fillings  and 
backings.  In  a  mixed  masonry  of  this  character  the  courses 
of  dressed  stone  may  project  slightly  beyond  the  surfaces  of 
the  rest  of  the  structure.  The  architectural  effect  of  this 
arrangement  is  in  some  degree  pleasing,  particularly  when 
the  joints  are  chamfered  ;  and  the  method  is  obviously  useful 
in  structures  of  this  kind,  as  protection  is  afforded  by  it  to  the 
surfaces  which,  from  the  nature  of  the  material,  or  the  char- 
acter of  the  work,  offer  the  least  resistance  to  the  destructive 
action  of  floating  bodies  Hydraulic  mortar  should  alone  be 
used  in  every  part  of  the  masonry  of  bridges. 


Fig.  144 — Elevation  M  and  plan  N,  showing  the  manner  of  arranging  the  embankments  of 
the  approaches,  when  the  head- walls  of  the  bridge  are  simply  prolonged. 

a,  a',  side  slope  of  embankment, 

b,  6',  dry  stone  facing  of  the  embankment  where  its  end  is  rounded  off,  forming  a  quarter  of  a 
cone  finish. 

/,  /',  flight  of  steps  for  foot-passengers  to  ascend  the  embankment. 

c,  c',  embankment  arranged  as  above,  but  simply  sodded. 

d,  d',  facing  of  dry  stone  for  the  side  slopes  of  the  banks. 

e,  e\  facing  of  the  bottom  of  the  stream. 


591.  Approaches.  The  approaches  should  be  so  made  as 
to  procure  an  easy  and  safe  access  to  the  bridge,  and  not  ob- 
struct unnecessarily  other  channels  of  communication. 

When  several  avenues  meet  at  a  bridge,  or  where  tlie  width 
of  the  roadway  of  a  direct  avenue  is  greater  than  that  of  the 
bridge,  the  approaches  are  made  by  gradually  widening  the 
outlet  from  the  bridge,  until  it  attains  the  requisite  width, 
by  means  of  wing- walls  of  any  of  the  usual  forms  that  may 


306 


CIVIL  ENGINEERING. 


suit  the  locality.  The  form  of  wing-wall  (Fig.  145)  present- 
ing a  concave  surface  outward  is  usually  preferred  when  suited 
to  the  locality,  both  for  its  architectural  effect  and  its  strength. 
When  made  of  dressed  stone  it  is  of  more  difficult  construc- 
tion and  more  expensive  than  the  plane  surface  wall. 


592.  Water-wings.  To  secure  the  natural  banks  near  the 
bridge,  and  the  foundations  of  the  abutments  from  the  action 
of  the  current,  a  facing  of  dry  stone  or  of  masonry  sliould  be 
laid  upon  the  slope  of  the  banks,  which  sliould  be  properly 
prepared  to  receive  it,  and  the  foot  of  the  facing  must  be  se- 
cured by  a  mass  of  loose  stone  blocks  spread  over  the  bed 
around  it,  in  addition  to  which  a  line  of  square-jointed  piles 
may  be  previously  driven  along  the  foot.  When  the  face  of 
the  abutment  projects  beyond  the  natural  banks,  an  embank- 
ment faced  with  stone  should  be  formed,  connecting  the  face 
with  points  on  the  natural  banks  above  and  below  the  bridge. 
By  this  arrangement,  termed  the  water-wings^  the  natural 
water-way  will  be  gradually  contracted  to  conform  to  that 
left  by  the  bridge. 

593.  Enlargement  ofWater-way.  In  the  full  centre  and 
oval  arches,  when  the  springing  lines  are  placed  low,  the 
spandrels  present  a  considerable  surface  and  obstruction  to 
the  current  during  the  higher  stages  of  the  water.  This  not 
only  endangers  the  safety  of  the  bridge,  by  the  accumulation 
of  drift-wood  and  ice  which  it  occasions,  but,  during  these 
epochs,  gives  a  heavy  appearance  to  the  structure.  To  rem- 
edy these  defects  the  solid  angle,  formed  by  the  heads  and 
the  soffit  of  the  arch,  may  be  truncated,  the  base  of  the  cunei- 
form-shaped mass  taken  away  being  near  the  springing  lines 


Fig.  145— Represents  an  elevation  M  and  plan 


N  of  a  curved  face  win  p- wall. 

A,  front  view  of  wing-wall. 

B,  B',  slope  of  embankment. 


STONE  BRrOGES. 


307 


of  the  arch,  and  its  apex  near  the  crown.  The  form  of  the 
detached  mass  may  be  variously  arranged.  In  the  bridge  of 
Neuilly,  which  is  one  of  the  first  where  this  expedient  was 
resorted  to,  the  surface,  marked  F,  (Figs.  133,  134)  left  by 
detaching  the  mass  in  question,  is  warped,  and  lies  between 
two  plane  curves,  the  one  an  arc  of  a  circle  n  traced  on  the 
head  of  the  bridge,  the  other  an  oval,  m  o  o  jp,  traced  on  the 
6ofiit  of  the  arch.  This  affords  a  funnel-shaped  water-way  to 
each  arch,  and,  during  high  water,  gives  a  light  appearance 
to  the  structure,  as  the  voussoirs  of  the  head  ring-course  have 
then  the  appearance  of  belonging  to  a  flat  segmental  arch. 

594.  General  Remarks.  The  architecture  of  stone  bridges 
has,  within  a  somewhat  recent  period,  been  carried  to  a  very 
high  degree  of  perfection,  both  in  design  and  in  mechanical 
execution.  France,  in  this  respect,  has  given  an  example  to 
the  world,  and  has  found  worthy  rivals  in  the  rest  of  Europe, 
and  particularly  in  Great  Britain.  Her  territory  is  dotted 
over  with  innumerable  fine  monuments  of  this  character, 
which  attest  her  solicitude  as  well  for  the  public  welfare  as 
for  the  advancement  of  the  industrial  and  liberal  arts.  For 
her  progress  in  this  branch  of  architecture,  France  is  mainly 
indebted  to  her  School  and  her  Corps  of  Fonts  et  Chaussees ; 
institutions  which,  from  the  time  of  her  celebrated  engineer 
Perronet,  have  supplied  her  with  a  long  line  of  names,  alike 
eminent  in  the  sciences  and  arts  which  pertain  to  the  profes- 
sion of  the  engineer. 

England,  although  on  some  points  of  mechanical  skill  per- 
taining to  the  engineer's  art  the  superior  of  France,  holds  the 
second  rank  to  her  in  the  science  of  her  engineers.  Without 
establishments  for  professional  training  corresponding  to 
those  of  France,  the  English  engineers,  as  a  body,  have,  until 
within  a  few  years,  labored  under  the  disadvantage  of  having 
none  of  those  institutions  which,  by  creating  a  common  bond 
of  union,  serve  not  only  to  diffuse  science  throughout  the 
whole  body,  but  to  raise  merit  to  its  proper  level,  and  frown 
down  alike,  through  an  enlightened  esjprit  de  corjps^  the  as- 
sumptions of  ignorant  pretension,  and  the  malevolence  of 
petty  jealousies. 

^  Among  the  works  of  this  class,  in  this  country,  may  be 
cited  the  railroad  bridge,  called  the  Thomas  Viaduct,  over 
the  Patapsco,  on  the  line  of  the  Baltimore  and  Washington 
railroad,  designed  and  built  by  Mr.  B.  H.  Latrobe,  the  engi- 
neer of  the  road.  This  is  one  of  the  few  existing  bridge 
structuiSfes  with  a  curved  axis.  The  engineer  has  very  hap- 
pily met  the  double  difiiculty  before  him,  of  being  obliged 


308 


CIVIL  ENGINEERING. 


to  adopt  a  curved  axis,  and  of  the  want  of  workmen  suffi- 
ciently conversant  with  the  application  of  working  drawings 
of  a  rather  complicated  character,  by  placing  full  centre 
cylindrical  arches  upon  piers  with  a  trapezoidal  horizontal 
section.  This  structure,  with  the  exception  of  some  minor 
details  in  rather  questionable  taste,  as  the  slight  iron  parapet 
railing,  for  example,  presents  an  imposing  aspect^  and  does 
great  credit  to  the  intelligence  and  skill  of  the  engineer  at 
the  time  of  its  construction,  but  recently  launched  in  a  new 
career.  The  fine  single  arch,  known  as  the  Carrolton  Via- 
duct,  on  the  Baltimore  and  Ohio  railroad,  is  also  highly 
creditable  to  the  science  and  skill  of  the  engineer  and  me- 
chanics under  whom  it  was  raised.  One  of  the  largest 
bridges  in  the  United  States,  designed  and  partly  executed 
in  stone,  is  the  JPotomac  Aqueduct  at  Georgetown,  where  the 
Chesapeake  and  Ohio  canal  intersects  the  Potomac  river. 
This  work,  to  which  a  wooden  superstructure  has  been  made, 
was  built  under  the  superintendence  of  Captain  Turnbull  of 
the  U.  S.  Topographical  Engineei*s. 

595.  The  following  table  contains  a  summary  of  the  prin- 
cipal details  of  some  of  the  more  noted  stone  bridges  of 
Europe : 


Name  of 
Bbldge. 

Eiver. 

1  Form  of  Arch. 

Number  of 
arches. 

Span  of  widest 
span. 

Rise. 

Depth  of  key- 
stone. 

Width  between 
the  heads. 

Date. 

Name  of  Engi- 
neer. 

Vieille-Brionde 

Allier. 

Segment. 

1 

178 

69 

5.3 

1454 

Grenier  &  Estone. 

1 

98.6 

23 

1578 

Michel  Angelo. 

Claix  

Drac. 

u 

1 

150 

54 

3!l 

1611 

Neuilly..  

Seine. 

Elliptical. 

5 

127.9 

31.9 

5.3 

47 '.9 

1774 

Perronet. 

Agout. 

1 

1(50.5 

65 

10.9 

1775 

Saget. 

Saint-Maxence 

Oise. 

Segment. 

3 

76.7 

6 

5 

4i!5 

1784 

Perronet. 

Erault. 

Elliptical. 

1 

160 

44 

6.5 

1793 

Garipny. 

Seine. 

Segment. 

5 

91.8 

10.8 

4.6 

43  ".7 

1811 

Lamand6. 

Seine. 

5 

101.7 

13.7 

4.6 

49.2 

1813 

Lamand6. 

Waterloo  

Thames. 

ElUptical. 

9 

120 

35 

4.9 

45 

1816 

Rennie. 

Gloucester  

Severn. 

1 

150 

54 

4.5 

35 

1827 

Telford. 

ThameH. 

(( 

6 

152 

37.8 

5 

56 

1831 

Rennie. 

Dora  Biparia. 

Segment, 

1 

147.6 

18 

4.9 

40 

Mosca. 

Grosvenor  

Dee. 

1 

200 

42 

4 

1833 

Hartley. 

596.  Among  the  recent  French  bridges,  presenting  some 
interesting  features  in  their  construction,  may  be  cited  that 
of  Souillac  over  the  Dordogne.  The  river  at  this  place  hav- 
ing a  torrent-like  character,  and  the  bed  being  of  lime-stone 
rock  with  a  very  uneven  surface,  and  occasional  deep  fissures 
filled  with  sand  and  gravel,  the  obstacle  to  using  either  the 
caisson,  or  the  ordinary  coffer-dam  for  the  founda^ns,  was 
very  great.    The  engineer,  M.  Yicat,  so  well  known  by  hi8 


STONE  BRIDGES. 


309 


researches  upon  mortar,  etc.,  devised,  to  obviate  these  difficul- 
ties, the  plan  of  enclosing  the  area  of  each  pier  by  a  coffer- 
work  accurately  fitted  to  the  surface  of  the  bed,  and  of  filling 
this  with  beton  to  form  a  bed  for  the  foundation  courses. 
This  he  effected,  by  first  forming  a  framework  of  heavy  tim- 
ber, so  arranged  that  thick  sheeting-piles  could  be  driven 
close  to  the  bottom,  between  its  horizontal  pieceSj  and  form  a 
well-jointed  vessel  to  contain  the  semi-fluid  material  for  the 
bed.  After  this  coffer-work  was  placed,  the  loose  sand  and 
gravel  was  scooped  from  the  bottom,  the  asperities  of  the 
surface  levelled,  and  the  fissures  were  voided,  and  refilled 
with  fragments  of  a  soft  stone,  which  it  was  found  could  be 
more  compactly  settled,  by  ramming,  in  the  fissures,  than  a 
looser  and  rounder  material  like  gravel.  On  this  prepared 
surface,  the  bed  of  beton,  which  was  from  12  to  15  feet  in 
thickness,  was  gradually  raised,  by  successive  layers,  to  with- 
in a  few  feet  of  the  low-water  level,  and  the  stone  superstruc- 
ture then  laid  upon  it,  by  using  an  ordinary  coffer-dam  that 
rested  on  the  framework  around  the  bed.  In  this  bridge,  as 
in  that  of  Bordeaux,  a  provisional  trial-weight,  greater  than 
the  permanent  load,  was  laid  upon  the  bed,  before  com- 
mencing the  superstructure. 

To  give  greater  security  to  foundations,  they  may  be  sur- 
rounded with  a  mass  of  loose  stone  blocks  thrown  in  and 
allowed  to  find  their  own  bed.  Where  piles  are  used  and 
project  some  height  above  the  bottom,  besides  the  loose  stone, 
a  grating  of  heavy  timber,  placed  between  and  enclosing  the 
piling,  may  be  used  to  give  it  greater  stiffuess  and  prevent 
outward  spreading.  In  streams  of  a  torrent  character,  where 
the  bed  is  liable  to  be  worn  away,  or  shifted,  an  artificial 
covering,  or  apron  of  stone  laid  in  mortar,  has,  in  some  cases, 
been  used,  both  under  the  arches  and  above  and  below 
the  bridge,  as  far  as  the  bed  seemed  to  require  this  protec- 
tion. At  the  bridge  of  Bordeaux  loose  stone  was  spread 
over  the  river-bed  between  the  piers,  and  it  has  been  found 
to  answer  perfectly  the  object  of  the  engineer,  the  blocks 
having,  in  a  few  years,  become  united  into  a  firm  mass  by 
the  claye}'  sediment  of  the  river  deposited  in  their  interstices. 
At  the  elegant  cast-iron  bridge,  built  over  the  Lary^  near 
Plymouth,  resort  was  had  to  a  similar  plan  for  securing  the 
bed,  which  is  of  shifting  sand.  The  engineer,  Mr.  Eendel, 
here  laid,  in  the  first  place,  a  bed  of  compact  clay  upon  the 
sand  bed  between  the  piers,  and  imbedded  in  it  loose  stone. 
This  method,  which  for  its  economy  is  worthy  of  note,  has 
fully  answered  the  expectations  of  the  engineer. 


310 


CIVIL  ENGINEEEING. 


III. 

WOODEN  BRIDGES. 

597.  Abutments.  The  abutments  and  piers  of  wooden 
bridges  may  be  either  of  stone  or  of  timber.  Stone  sup- 
ports are  preferable  to  those  of  timber,  both  on  account  of 
the  superior  durability  of  stone,  and  of  its  offering  more 
security  than  frames  of  timber  against  the  accidents  to  which 
the  piers  of  bridges  are  liable  from  freshets,  ice,  &c. 

598.  Wooden  abutments  may  be  formed  by  constructing 
what  is  termed  a  crih-work,  which  consists  of  large  pieces  of 
square  timber  laid  horizontally  upon  each  other,  to  form  the 
upright  or  sloping  faces  of  the  abutment.  These  pieces  are 
halved  into  each  other  at  the  angles,  and  are  otherwise  firmly 
connected  together  by  diagonal  ties  and  iron  bolts.  The  space 
enclosed  by  the  crib-work,  which  is  usually  built  up  in  the 
manner  just  described,  only  on  three  sides,  is  filled  with  earth 
carefully  rammed,  or  with  dry  stone,  as  circumstances  may 
seem  to  require. 

A  wooden  abutment  of  a  more  economical  construction 
may  be  made,  by  partly  imbedding  large  beams  of  timber 
placed  in  a  vertical  or  an  inclined  position,  at  intervals  of  a  few 
feet  from  each  other,  and  forming  a  facing  of  thick  plank  to 
sustain  the  earth  behind  the  abutment.  Wooden  piers  may 
also  be  made  according  to  either  of  the  methods  here  laid 
down,  and  be  filled  with  loose  stone,  to  give  them  sufficient 
stability  to  resist  the  forces  to  which  they  may  be  exposed  ; 
but  the  method  is  clumsy,  and  inferior,  under  every  point  of 
view,  to  stone  piers,  or  to  the  methods  which  are  about  to  be 
explained. 

599.  The  simplest  arrangement  of  a  wooden  pier  consists 
(Fig.  146)  in  driving  heavy  square  or  round  piles  in  a  single 
row,  placing  them  from  two  to  four  feet  apart.  These  upright 
pieces  are  sawed  off  level,  and  connected  at  top  by  a  horizon- 
tal beam,  termed  a  ca^,  which  is  either  mortised  to  receive  a 
tenon  made  in  each  upright,  or  else  is  fastened  to  the  uprights 
by  bolts  or  pins.  Other  pieces,  which  are  notched  and  bolted 
in  pairs  on  the  sides  of  the  uprights,  are  placed  in  an  inclined 
or  diagonal  position,  to  brace  the  whole  system  firmly.  The 
several  uprights  of  the  pier  are  placed  in  the  direction  of  the 
thread  of  the  current.  If  thought  necessary,  two  horizontal 
beams,  arranged  like  the  diagonal  pieces,  may  be  added  to 
tlie  system  just  below  the  lowest  water-level.  In  a  pier  of 
this  kind,  the  place  of  the  starlings  is  supplied  by  two  in- 


PILE  FOUNDATIONS. 


clined  beams  on  the  same  Hue  with  the  uprights,  which  are 
termed  fender-heams. 


a 


vVvvVvVvvVvv 

Fig.  146 — Elevation  of  a  wooden  pier, 

a,  a,  piles  of  substructure. 

6,  6,  capping  of  piles  arranged  to  receive  the  ends  of  the  uprights  c,  c,  which  support  the 

string-pieces  i,  i, 
d,  upper  fender  beam. 
«,  lower  fender  beam. 

/■,  horizontal  ties  bolted  in  pairs  on  the  uprights. 

g,  diagonal  braces  bolted  in  pairs  on  the  uprights. 

h,  capping  of  the  uprights  placed  under  the  string  pieces. 

A,  roadway. 

B,  parapet. 

600.  A  strong  objection  to  the  system  just  described,  arises 
from  the  difficulty  of  replacing  the  uprights  when  in  a  state 
of  decay.    To  remedy  this  defect,  it  has  been  proposed  to 


Fig.  147 — Plan  0,  elevation  M,  and  cross  section  N, 
showing  the  arrangement  of  the  capping  of  the 
foundation  piles  with  the  uprights. 

a,  piles. 

&,  capping  of  four  beams  bolted  together, 
c,  uprights. 


drive  large  piles  in  the  positions  to  be  occupied  by  the  uprights 
(Fig.  147),  to  connect  these  piles  below  the  low-water  level 
by  four  horizontal  beams,  firmly  fastened  to  the  heads  of  the 
piles,  which  are  sawed  off  at  a  proper  height  to  receive  the 


312 


CIVIL  ENGINEEEING. 


horizontal  beams.  The  two  top  beams  have  large  square 
mortises  to  receive  the  ends  of  the  uprights,  which  rest  on 
those  of  the  piles.  The  rest  of  the  system  may  be  construct- 
ed as  in  the  former  case.  By  this  arrangement  the  uprightsjl 
when  decayed,  can  be  readily  replaced,  and  they  rest  on  a 
solid  substructure  not  subject  to  decay ;  shorter  timber  also 
can  be  used  for  the  piers  than  when  the  uprights  are  driven 
into  the  bed  of  the  stream. 

^  601.  In  deep  water,  and  especially  in  a  rapid  current,  a 
single  row  of  piles  might  prove  insufficient  to  give  stability 
to  the  iiprights ;  and  it  has  therefore  been  proposed  to  give 
a  sufficient  spread  to  the  substructure  to  admit  of  bracing  the 
uprights  by  struts  on  the  two  sides.  To  efPect  this,  three 
piles  (Fig.  148)  should  be  driven  for  each  upright ;  one  just 
under  its  position,  and  the  other  two  on  each  side  of  this,  on 
a  line  perpendicular  to  that  of  the  pier.  The  distance  be- 
tween the  three  piles  will  depend  on  the  inclination  and 
length  that  it  may  be  deemed  necessary  to  give  the  struts.  The 
heads  of  the  three  piles  are  sawed  off  level,  and  connected 
by  two  horizontal  clamping  pieces  below  the  lowest  water. 


HUH 


Fier,  148 — Elevation  of  the  arrangement  of  a  wide 

foundation  for  a  wooden  pier. 
a,  upright. 

ft,  6,  piles  of  the  foundation. 
Cy  c.  capping  of  the  piles. 

d,  d,  struts  to  strengthen  the  uprights. 

e,  e,  clamping  pieces  bolted  in  pairs  on  tbe  uprighta. 


A  square  mortise  is  left  in  these  two  pieces,  over  the  middle 
pile,  to  receive  the  uprights.  The  uprights  are  fastened  to- 
gether at  the  bottom  by  two  clamping  pieces,  which  rest  on 
those  that  clamp  the  heads  of  the  piles,  and  are  rendered 
firmer  by  the  two  struts. 

602.  In  localities  where  piles  cannot  be  driven,  the  uprights 
of  the  piers  may  be  secured  to  the  bottom  by  means  of  a  gra- 
ting, arranged  in  a  suitable  manner  to  receive  the  ends  of  the 
uprights.  The  bed,  on  which  the  grating  is  to  rest,  having 
been  suitably  prepared,  it  is  floated  to  its  position,  and  sunk 
either  before  or  after  the  uprights  are  fastened  to  it,  as  may 
be  found  most  convenient.  The  grating  is  retained  in  its 
place  by  loose  stone.  As  a  farther  security  for  the  piers,  the 
uprights  may  be  covered  by  a  sheathing  of  boards,  and  the 
spaces  between  the  sheathing  be  filled  in  with  gravel. 

603.  As  wooden  piers  are  not  of  a  suitable  form  to  resist 


PILE  FOUNDATIONS. 


313 


heavy  shocks,  ice-breakers  should  be  placed  in  the  stream, 
opposite  to  each  pier,  and  at  some  distance  from  it.  In 
streams  with  a  gentle  current,  a  simple  inclined  beam  (Fig. 
149)  covered  v^rith  thick  sheet-iron,  and  supported  by  uprights 


and  diagonal  pieces,  will  be  all  that  is  necessary  for  an  ice- 
breaker. But  in  rapid  currents  a  crib-work,  having  the  form 
of  a  triangular  pyramid  (Fig.  150),  the  up-stream  edge  of 


Fig.  150— Elevation  M  and  plan  N  of  the 
frame  of  an  ice-breaker  to  be  filled  in 
with  broken  stone. 


which  is  covered  with  iron,  will  be  required,  to  offer  sufficient 
resistance  to  shocks.  The  crib-work  may  be  filled  in,  if  it  be 
deemed  advisable,  with  blocks  of  stone. 

604.  In  determining  the  length  of  the  span  the  engineer 
must  take  into  consideration  the  fact  that  wooden  bridges 
require  more  frequent  repairs  than  those  of  stone,  arising 


314 


CIVIL  ENGINEERING. 


from  the  decay  of  the  material,  and  from  the  effects  of  shrink- 
ing and  vibrations  upon  the  joints  of  the  frames,  and  that  the 
difficulty  of  replacing  decayed  parts,  and  readjusting  the 
framework,  increases  rapidly  with  the  span. 

605.  Bridge- frames  may  be  divided  into  two  general  classes. 
To  the  one  belong  all  those  combinations,  whether  of  straight  or 
of  curved  timber,  that  exert  a  lateral  pressure  upon  the  abut- 
ments and  piers,  and  in  which  the  superstructure  is  generally 
above  the  bridge-frame.  To  the  other,  those  combinations 
which  exert  no  lateral  pressure  upon  the  points  of  support, 
and  in  which  the  roadway,  &c.,  may  be  said  to  be  suspended 
from  the  bridge-frame. 

606.  Definitions  of  some  of  the  terms  employed  in 
bridge  nomenclature. 

A  Chord  is  the  upper  or  lower  member  in  a  truss.  It  ex- 
tends from  end  to  end  of  the  structure.  There  are  usually  two 
chords,  an  upper  and  a  lower  chord.  These  may  be  parallel, 
as  in  Figs.  157  and  167,  or  the  upper  one  may  be  curved 
(arched)  and  the  lower  one  horizontal,  or  both  may  be  curved. 
These  pieces  by  some  English  writers  are  called  booms,  and  by 
others  stringers.  The  lower  chord  is  often  called  a  tie.  The 
upper  chord  is  sometimes  called  a  strcmiing  beam. 

A  Tie  is  a  piece  which  connects  two  parts  and  is  subjected 
to  tension. 

A  Strut  is  a  general  term  which  is  applied  to  a  piece  in  a 
truss  which  is  subjected  to  compression.  In  proportioning  it, 
it  is  treated  as  a  pillar.  In  its  more  restricted  sense,  it  is  a 
short  piece  which  is  subjected  to  compression. 

A  Tie-Strut,  or  Strut- Tie,  is  a  piece  which  may  be  sub- 
jected to  tension  and  compression  at  different  times,  under 
different  conditions  of  loading. 

A  Bracei^  an  inclined  piece  which  is  subjected  to  compres- 
sion. It  is  an  inclined  strut.  In  bridges,  braces  are  some- 
times distinguished  as  main-hraces  and  counter-braces.  This 

Fig.  151. 


WOODEN  BRIDGES. 


315 


distinction  is  quite  unnecessary  in  an  analytical  point  of  view, 
as  will  be  seen  hereafter,  but  it  is  so  common  in  practice  that 
it  will  not  do  to  ignore  it. 

A  Main-Brace  is  a  brace  which  inclines  from  the  end  of  a 
truss  towards  the  centre,  as  in  Fig.  151. 

A  Counter -Br ace  is  one  which  inclines  from  the  centre 
and  towards  the  ends.  In  the  same  panel  the  counter-brace 
inclines  the  opposite  way  from  the  main-brace.  See  Fig.  151. 

A  Tie-Brace  performs  the  office  of  both  main  and  counter- 
brace  ;  it  is  ihe  same  as  a  Tie-Strut. 

607.  Long's  Truss.  This  was  one  of  the  first  trusses  of 
this  country  in  which  a  scientific  arrangement  of  the  parts 
was  observed.  It  was  composed  entirely  of  wood,  even  iron 
bolts  for  splicing  the  main  beams  being  avoided.  It  consists 
in  forming  both  the  upper  and  lower  beams  (Fig.  152)  of 
three  parallel  beams,  sufficient  space  being  left  between  the 


Fig.  152 — Represents  a  panel  of  Long's  truss. 
A  and  B,  top  and  bottom  strings  of  three  courses. 
0,  C,  posts  in  pairs. 

D,  braces  in  pairs. 

E,  counter-brace  single. 

a,  a,  mortises  where  jibs  and  keys  are  inserted. 

F,  jib  and  key  of  hard  wood. 


316 


CIVIL  ENGINEERING. 


one  in  the  centre  and  the  other  two  to  insert  the  cross  pieces, 
termed  the  ^osts  /  the  posts  consist  of  beams  in  pairs  placed 
at  suitable  intervals  along  the  strings,  with  which  they  are 
connected  by  wedge  blocks,  termed  jibs  and  keys,  which  are 
inserted  into  rectangular  holes  made  through  the  strings, 
and  fitting  a  corresponding  shallow  notch  cut  into  each  post. 
A  brace  connects  the  top  of  one  post  with  the  foot  of  the 
one  adjacent  by  a  suitable  joint.  Another  diagonal  piece, 
termed  the  counter-brace,  is  placed  crosswise  between  the  two 
braces  and  their  posts,  with  its  ends  abutting  against  the 
centre  beam  of  the  upper  and  lower  strings.  The  counter- 
braces  are  connected  with  the  posts  and  braces  by  wooden 
pins,  termed  tree-nails. 

In  wide  bearings,  the  strings  require  to  be  made  of  several 
beams  abutting  end  to  end ;  in  this  case  the  beams  should 
break  joints,  and  short  beams  should  be  inserted  between  the 
centre  and  exterior  beams  wherever  the  joints  occur,  to 
strengthen  them. 

The  beams  in  this  combination  are  all  of  uniform  cross 
section,  the  joints  and  fastenings  are  of  the  simplest  kind, 
and  the  parts  are  well  distributed  to  call  into  play  the 
strength  of  the  strings,  and  to  produce  uniform  stiffness  and 
strain. 

608.  Town's  Truss. — The  combination  of  Mr.  Town 
(Fig.  153)  consists  in  two  main  strings,  each  formed  of  two  or 


Fig  153.— Represents  an  elevation  A,  and  end  view,  B, 

of  a  portion  of  Town's  truss, 
o,  a,  top  strings. 
6,  6,  bottom  strings, 
c,  c,  diagonal  braces. 


three  parallel  beams  of  two  thicknesses  breaking  joints.  Be- 
tween the  parallel  beams  are  inserted  a  series  of  diagonal 
beams  crossing  each  other.  These  diagonals  are  connected 
with  the  strings  and  with  each  other  by  tree-nails.  When 
the  strings  are  formed  of  three  parallel  beams,  diagonal 
pieces  are  placed  between  the  centre  and  exterior  beams,  and 
two  intermediate  strings  are  placed  between  the  two  courses 
of  diagonals. 

This  combination,  commonly  known  as  the  lattice  truss,  is 
of  very  easy  mechanical  execution,  the  beams  being  of  a  uni- 


WOODEN  BRIDGES. 


317 


form  cross  section  and  length.  The  strains  upon  it  are  borne 
by  the  tree-nails,  and  when  used  for  structures  subjected  to 
variable  strains  and  jars,  it  loses  its  stiffness  and  sags  between 
the  points  of  support.  It  is  more  commendable  for  its 
simplicity  than  scientific  combination. 

609.  Howe's  Truss. — This  truss  consists  of  (Fig.  154)  an 
upper  and  lower  string,  each  formed  of  several  thicknesses 
of  beams  placed  side  by  side  and  breaking  joints.  On  the 
upper  side  of  the  lower  string  and  the  lower  side  of  the 
upper,  blocks  of  hard  wood  are  inserted  into  shallow  notches ; 
the  blocks  are  bevelled  off  on  each  side  to  form  a  suitable 
point  of  support,  or  stej[>  for  the  diagonal  pieces.  One  series 
of  the  diagonal  pieces  are  arranged  in  pairs,  the  others  are 
single  and  placed  between  those  in  pairs.  Two  strong  bolts 
of  iron,  which  pass  through  the  blocks,  connect  the  upper 
and  lower  strings,  and  are  arranged  with  a  screw  cut  on  one 
end  and  a  nut  to  draw  the  parts  closely  together. 

This  combination  presents  a  judicious  arrangement  of  the 
parts.  The  blocks  give  abutting  surfaces  for  the  braces  su- 
perior to  those  obtained  by  the  ordinary  forms  of  joint  for 
this  purpose.    The  bolts  replace  advantageously  the  timber 


posts,  and  in  case  of  the  frame  working  loose  and  sagging, 
their  arrangement  for  tightening  up  the  parts  is  simple  and 
efficacious. 

610.  Schuylkill  Bridge.— This  bridge,  designed  and 
built  by  L.  Wernwag,  has  the  widest  span  of  any  wooden 


S18 


CIVIL  ENGINEERING. 


Fig  86  is  a  perspective  view  of  a  part  of  one  paneB 
of  the  Howe  truss,  and  shows  quite  clearly  how 
the  parts  are  arranged.  For  an  analysis  ofi 
these  structures,  tsee  Wood's  Treatise  on  Bridges 
and  Roofs,  j 


bridge  in  this  country.  The  bridge-frame  (Fig.  155)  consisted 


Fig.  155 — Represents  a  side  view  of  a  portion  of  the  open-curved  rib  of  the 
bridge  over  the  Schuylkill  at  Philadelphia. 

A,  lower  curved  built  beam. 

B,  top  beam, 
a,  fl,  posts. 

c,  c,  diagonal  braces, 
o,  o,  iron  diagonal  ties. 

m,  m,  iron  stays  anchored  in  the  abutment  C. 

of  five  ribs.  Each  rib  is  an  open-built  beam  formed  of  a 
bottom  curved  solid-built  beam  and  of  a  single  top  beam, 
which  are  connected  by  radial  pieces,  diagonal  braces,  and 
inclined  iron  stays.  The  bottom  curved  beam  is  composed 
of  three  concentric  solid-built  beams,  slightly  separated  from 
each  other,  each  of  which  has  seven  courses  of  curved  scant- 
ling in  it,  each  course  6  inches  thick  by  13  inches  in  breadth ; 
the  courses,  as  well  as  the  concentric  beams,  being  firmly 
united  by  iron  bolts,  &:c.  A  roadway  that  rests  upon  the 
bottom  curved  ribs  is  left  on  each  side  of  the  centre  rib,  and 
a  footpath  between  each  of  the  two  exterior  ribs.  The  bridge 
was  covered  in  by  a  roof  and  a  sheathing  on  the  sides. 

611.  Burr's  Truss. — Burr's  plan,  which  (Fig.  156)  consists 


WOODEN  BRIDGES. 


319 


in  forming  each  rib  of  an  open-built  beam  of  straight  timber, 
and  connecting  with  it  a  curved  solid-built  beam  formed  of 
two  or  more  thicknesses  of  scantling,  between  which  the 


Fig,  156 — Eepresents  a  side  view  of  a 

portion  of  a  rib  of  Burr's  bridge, 
a,  a,  arch  timbers, 
d,  d,  queen-posts. 
&,  6,  braces, 
c,  c,  chords. 


e,  e,  plate  of  the  side  frame. 

o,  o,  floor  girders  on  which  the  flooring  ; 

joists  and  flooring  boards  rest. 
n,  n,  check  braces. 

t,  tie-beams  of  roof. 
A,  portion  of  pier. 


framework  of  the  open-built  beam  is  clamped.  The  open- 
built  beam  consists  of  a  horizontal  bottom  beam  of  two 
thicknesses  of  scantling,  termed  the  chords,  between  which 
are  secured  the  nprights,  termed  the  queen  posts, — of  a  single 
top  beam,  termed  the  plate  of  the  side  frame,  which  rests 
upon  the  uprights,  with  which  it  is  connected  by  a  mortise 
and  tenon  joint, — and  of  diagonal  braces  and  other  smaller 
braces,  termed  check  hraces,  placed  between  the  uprights. 


The  curved-built  beam,  termed  the  arch-timbers,  is  bolted 
upon  the  timbers  of  the  open-built  beam.    The  bridge-frame 


320 


CIVIL  ENGINEEEINQ. 


may  consist  of  two  or  more  ribs,  which  are  connected  and 
stiffened  by  cross  ties  and  diagonal  braces.  The  roadway- 
flooring  is  laid  npon  cross  pieces,  termed  the  floor  girders^ 
which  may  either  rest  upon  the  chords,  or  else  be  attached  at 
any  intermediate  point  between  them  and  the  top  beam. 
The  roadway  and  footpaths  may  be  placed  in  any  position 
between  the  several  ribs. 

612.  Pratt's  Truss.  This  truss  (Fig.  157)  has  the  same 
general  form  as  Howe's,  but  differs  in  its  details.  The  ver- 
ticals here  are  wooden  posts  instead  of  iron  rods,  and  the 
diagonals  are  iron  ties  instead  of  wooden  braces. 

613.  MeCallum's  Truss.   This  truss  (Fig.  158)  is  a  modi- 


rig.  158. 


fication  of  Howe's,  the  essential  difference  of  which  consists 
in  a  curved  upper  chord  instead  of  a  horizontal  one.  The 
long  braces  at  the  end — called  arch  hraces, — are  not  essential 
to  this  system.  This  system  is  stiffer  than  similar  ones  having 
horizontal  chords. 

614.  A  simple  but  effective  structure,  shown  in  Fig.  159, 


Fig.  159. 


WOODEN  BEIDGES. 


321 


has  been  in  use  for  some  time  on  the  K.  Y.  State  canals  for 
common  road  bridges,  and  for  crossings  on  farms.  There 
are  no  counter-braces,  which,  as  may  readily  be  shown,  are 
unnecessary  for  short  spans.  (See  Wood's  Treatise  on  Bridges 
and  Hoofs,  pp.  120  and  121.)  The  lower  timber  may  be 
spliced,  or 'in  any  other  manner  made  continuous  throughout. 
Another  timber,  which  is  placed  on  this,  extends  over  two  or 
four  of  the  central  bays.  The  verticals,  which  are  iron  rods, 
are  made  divergent,  as  shown  in  Fig.  159^&. 


159  a.   Cross  section  of  a  New  York  State 
canal  bridge. 

A,  upper  chord. 

B,  lower  chord, 

a,  6,  suspending  rods,  which  incline  out- 
ward. 

C,  a  floor-girder. 
d,  a  diagonal  rod. 


615.  Wooden  Arches.  A  wooden  arch  may  be  formed  by 
bending  a  single  beam  (Fig.  160)  and  confining  its  extremi- 


Pig.  160  — Represents  a  horizontal 
beam  c  supported  at  its  middle 
point  by  a  bent  beam  b. 


ties  to  prevent  it  Jrom  resuming  its  original  shape.  A  beam 
in  this  state  presents  greater  resistance  to  a  cross  strain  than 
when  straight,  and  may  be  used  with  advantage  where  great 
stiffness  is  required,  provided  the  points  of  support  are  of 
sufficient  strength  to  resist  the  lateral  thrust  of  the  beam. 
This  method  can  be  resorted  to  only  in  narrow  bearings. 

For  wide  arches  a  curved-built  beam  must  be  adopted ;  and 
for  this  purpose  a  solid  (Figs.  161  and  162)  or  an  open-built 
beam  may  be  used,  depending  on  the  bearing  to  be  spanned 
by  the  arch.    In  either  case  the  curved  beams  are  built  in 
21 


322 


CIVIL  ENGINEERING. 


the  same  manner  as  straight  beams,  the  pieces  of  which  they 
are  formed  being  suitabl}^  bent  to  conform  to  the  curvature 
of  the  arch,  which  may  be  done  either  by  steaming  the  pieces, 
by  mechanical  power,  or  by  the  usual  method  of  softening  the 
woody  fibres  by  keeping  the  pieces  wet  while  subjected  to 


the  heat  of  a  light  blaze. 


Fig.  161. 


Fig.  161 — Represents  a  wooden  arch  A,  formed  of  a  solid-built  beam  of  three 
courses,  which  support  the  beams  c,  c  by  the  posts  6f,  which  are  formed 
of  pieces  in  pairs. 

6,  6,  inclined  struts  to  strengthen  the  arch  by  relieving  it  of  a  part  of  the 
load  on  the  beams  c,  c. 


Fig.  162. 


a. 

 i 

a 

B 

0 

K 

1  L 

 i 

J  L 

Fig.  162— Represents  a  wooden  arch  of  a  solid-built  beam  A,  which  supports 
the  horizontal  beam  B  by  means  of  the  posts  a,  a.  The  arch  is  let  into 
the  beam  B,  which  acts  as  a  tie  to  confine  its  extremities. 


616.  The  number  of  ribs  in  the  bridge-frame  will  depend 
on  the  general  strength  required  by  the  object  of  the  struc- 
ture, and  upon  the  class  of  frame  adopted.  In  the  first 
class,  in  which  the  roadway  is  usually  above  the  frames,  any 
requisite  numl>er  of  ribs  may  be  used,  £uid  they  may  be 
placed  at  equal  intervals  apart,  or  else  be  so  placed  as  to  give 
the  best  support  to  the  loads  which  pass  over  tlie  bridge.  In 
the  second  class,  as  the  frame  usually  lies  entirely,  or  projects 
partly  above  the  roadway,  &c.,  if  more  than  two  ribs  are  re- 
quired, they  are  so  arranged  that  one  or  two,  as  circumstances 
may  demand,  form  each  head  of  the  bridge,  and  one  or  two 
more  are  placed  midway  between  the  heads,  so  as  to  leave  a 
sufticient  width  of  roadway  between  the  centre  and  adjacent 
ribs.    The  footpaths  are  usually,  in  this  case,  either  placed 


WOODEN  BRIDGES. 


323 


between  the  two  centre  ribs,  or,  when  there  are  two  exterior 
ribs,  between  them. 

617.  In  frames  which  exert  a  lateral  pressure  against  the 
abutments  and  piers,  the  lowest  points  of  the  framework 
should  be  so  placed  as  to  be  above  the  ordinary  high-water 
level ;  and  plates  of  some  metal  should  be  inserted  at  those 
points,  both  of  the  frame  and  of  the  supports,  where  the 
effect  of  the  pressure  might  cause  injury  to  the  woody  fibre. 

618.  The  roadway  usually  consists  of  a  simple  flooring 
formed  of  cross  joists,  termed  the  roadway-hearers^  or  floor- 
girders^  and  flooring-boards,  upon  which  a  road-covering 
either  of  wood  or  stone  is  laid.  A  more  common  and  better 
arrangement  of  the  roadway,  now  in  use,  consists  in  laying 
longitudinal  joists  of  smaller  scantling  upon  the  roadway- 
bearers,  to  support  the  flooring-boards.  This  method  pre- 
serves more  effectually  than  the  other  the  roadway-bearers 
from  moisture.  Besides,  in  bridges  which,  from  the  position 
of  the  roadway,  do  not  admit  of  vertical  diagonal  braces  to 
stiffen  the  framework,  the  only  means,  in  most  cases,  of 
effecting  this  object  is  in  placing  horizontal  diagonal  braces 
between  each  pair  of  road  way -bearers.  For  like  reasons, 
stone  road-coverings  for  wooden  bridges  are  generally  re- 
jected, and  one  of  plank  used,  which,  for  a  horse-track,  should 
be  of  two  thicknesses,  so  that,  in  case  of  repairs,  arising  from 
the  Avear  and  tear  of  travel,  the  boards  resting  upon  the 
flooring-joists  may  not  require  to  be  removed.  The  footpaths 
consist  simply  of  a  slight  flooring  of  sufficient  width,  which 
is  usually  detached  from  and  raised  a  few  inches  above  the 
roadway  surface. 

619.  When  the  bridge-frame  is  beneath  the  roadway,  a 
distinct  parapet  will  be  requisite  for  the  safety  of  passengers. 
This  may  be  formed  either  of  wood,  of  iron,  or  of  the  two 
combined.  It  is  most  generally  made  of  timber,  and  con- 
sists of  a  hand  and  foot  rail  connected  by  upright  posts  and 
stiffened  by  diagonal  braces.  A  wooden  parapet,  besides  the 
security  it  gives  to  passengers,  may  be  made  to  add  both  to 
the  strength  and  stiffness  of  the  bridge,  by  constructing  it  of 
timber  of  a  suitable  size,  and  connecting  it  firmly  with  the 
exterior  ribs. 

620.  In  bridge-frames  in  which  the  ribs  are  above  the  road- 
way, a  timber  sheathing  of  thin  boards  will  be  requisite  on  the 
sides,  and  a  roof  above,  to  protect  the  structure  from  the 
weather.  The  tie-beams  of  the  roof -trusses  may  serve  also  as 
ties  for  the  ribs  at  top,  and  may  receive  horizontal  diagonal 
braces  to  stiffen  the  structure,  like  those  of  the  roadway- 


324 


CIVIL  ENGINEERING. 


bearers.  The  rafters,  in  the  case  in  which  there  is  no  centre 
rib,  and  the  bearing,  or  distance  between  the  exterior  ribs,  is 
so  great  that  the  roadway-bearers  require  to  be  supported  in 
the  middle,  may  serve  as  points  of  support  for  snsj)ension 
pieces  of  wood,  or  of  iron,  to  which  the  middle  point  of  the . 
roadway-bearers  may  be  attached. 

621.  The  frame  and  other  main  timbers  of  a  wooden  bridge 
will  not  require  to  be  coated  with  paint,  or  any  like  compo- 
sition, to  preserve  them  from  decay  when  they  are  roofed 
and  boarded  in  to  keep  them  dry.  When  this  is  not  the  case, 
the  ordinary  preservatives  against  atmospheric  action  may  be 
used  for  them.  The  imder  surface  and  joints  of  the  planks 
of  the  roadway  may  be  coated  with  bituminous  mastic  when 
used  for  a  horse-track ;  in  railroad  bridges  a  metallic  cover- 
ing may  be  suitably  used  when  the  bridge  is  not  traversed  by 
horses. 

622.  Wooden  bridges  can  produce  but  little  other  archi- 
tectural effect  than  that  which  naturally  springs  up  in  the 
mind  of  an  educated  spectator  in  regarding  any  judiciously- 
contrived  structure.  When  the  roadway  and  parapet  are 
above  the  bridge-frame,  a  very  simple  cornice  may  be  formed 
by  a  proper  combination  of  the  roadway-timbers  and  flooring, 
which,  with  the  parapet,  will  present  not  only  a  pleasing  ap- 
pearance to  the  eye,  but  will  be  of  obvious  utility  in  covering 
the  parts  beneath  from  the  weather.  In  covered  bridges,  the 
most  that  can  be  done  will  be  to  paint  them  with  a  uniform 
coat  of  some  subdued  tint.  At  best,  from  their  want  of 
height  as  compared  with  their  length,  covered  wooden  bridges 
must,  for  the  most  part,  be  only  unsightly,  and  also  apparent- 
ly insecure  structures  when  looked  at  from  such  a  point  of 
view  as  to  embrace  all  the  parts  in  the  field  of  vision ;  and 
any  attempt,  therefore,  to  disguise  their  true  character,  and 
to  give  them  by  painting  the  appearance  of  houses,  or  of  stone 
arches,  while  it  must  fail  to  deceive  even  the  most  ignorant, 
will  only  betray  the  bad  taste  of  the  architect  to  the  more  en- 
lightened judge. 

The  art  of  erecting  wooden  bridges  has  been  carried  to 
great  perfection  in  almost  every  part  of  the  world  where 
timber  has,  at  any  period,  been  the  principal  building  mate- 
rial at  the  disposal  of  the  architect ;  but  iron  at  the  present 
day  is  fast  taking  the  place  of  wood  in  the  more  important 
bridges. 

623.  The  following  Table  contains  the  principal  dimen- 
sions of  some  of  the  most  celebrated  American  and  European 
wooden  bridges : 


WOODEN  BKIDGES. 


325 


NAME,  ETC.,  OF  BBIDGE. 

Number  of 
bays. 

Width  of 
Widest  biiy. 

Rise  or  depth 

1 

390  ft. 

— 

3 

193  " 

— 

1 

166  " 

— 

1 

308  " 

16.9  ft. 

3 

153  " 

11.6  " 

Essex  brid-O'e 

1 

350  " 

1 

340 

30  " 

3 

195  " 

13  " 

5 

300  " 

37  " 

29 

300  " 

19 

153  " 

15.4  " 

7 

180  " 

18  " 

Susquehanna  bridge  

10 

850  " 

CAST-IKON  BRIDGES. 

624.  Bridges  of  cast  iron  admit  of  even  greater  bold- 
ness of  design  than  those  of  timber,  owing  to  the  superiority, 
both  in  strength  and  durability,  of  the  former  over  the 
latter  material ;  and  they  may  therefore  be  resorted  to  under 
circumstances  very  nearly  the  same  in  which  a  wooden  struc- 
ture would  be  suitable. 

625.  The  abutments  and  piers  of  cast-iron  bridges  should 
be  built  of  stone,  as  the  corrosive  action  of  salt  water,  or 
even  of  fresh  water  when  impure,  would  in  time  render 
iron  supports  of  this  character  insecure ;  and  timber,  when 
exposed  to  the  same  destructive  agents,  is  still  less  durable 
than  cast  iron. 

626.  The  curved  ribs  of  cast-iron  bridge-frames  have  under- 
gone various  modifications  and  improvements.  In  the  earlier 
bridges,  they  w^ere  formed  of  several  concentric  arcs,  or 
curved  beams,  placed  at  some  distance  asunder,  and  united 
by  radial  pieces  ;  the  spandrels  being  filled  either  by  con- 
tiguous rings,  or  by  vertical  pieces  of  cast  iron  upon  which 
the  roadway  bearers  were  laid. 

In  the  next  stage  of  progress  towards  improvement,  the 
curved  ribs  were  made  less  deep,  and  were  each  formed  of 
several  segments,  or  panels  cast  separately  in  one  piece,  each 
panel  consisting  of  three  concentric  arcs  connected  by  radial 
pieces,  and  having  flanches,  with  other  suitable  arrangements, 
for  connecting  them  firmly  by  wrought-iron  keys,  screw-bolts, 
&c.  ;  the  entire  rib  thus  presenting  the  appearance  of  three 
concentric  arcs  connected  by  radial  pieces.  Tlie  spandrels 
were  filled  either  with  panels  formed  like  those  of  the  curved 


326 


CrVIL  ENGINEEEING. 


ribs,  with  iron  rings,  or  with  a  lozenge-shaped  reticulated 
combination.  The  ribs  were  connected  by  cast-iron  plates 
and  wrought-iron  diagonal  ties. 

In  the  more  recent  structures,  the  ribs  have  been  com- 
posed of  voussoir-shaped  panels,  each  formed  of  a  solid  thin 
plate  witli  flanches  around  the  edges  ;  or  else  of  a  curved 
tubular  rib,  formed  like  those  of  Polonceau,  or  of  Dela- 
field,  described  further  on.  The  spandrel-filling  is  either  a 
reticulated  combination,  or  one  of  contiguous  iron  rings. 
The  ribs  are  usually  united  by  cast-iron  tie-plates,  and 
braced  by  diagonal  ties  of  cast  and  wrought  iron. 

609.  The  roadway-bearers  and  flooring  may  be  formed 
either  of  timber,  or  of  cast  iron.  In  the  more  recent  struc- 
tures in  England,  they  have  been  made  of  the  latter  material ; 
the  roadway-bearers  being  cast  of  a  suitable  form  for  strength, 
and  for  their  connection  with  the  ribs;  and  the  flooring- 
plates  being  of  cast-iron. 

The  roadway  and  footpaths,  formed  in  the  usual  manner, 
rest  upon  the  flooring-plates. 

The  parapet  consists,  in  most  cases,  of  a  light  combina- 
tion of  cast  or  wrought  iron,  in  keeping  with  the  general 
style  of  the  structure. 

627.  The  English  engineers  have  taken  the  lead  in  this 
branch  of  architecture,  and,  in  their  more  recent  structures, 
have  carried  it  to  a  high  degree  of  mechanical  pei-fection 
and  architectural  elegance.  Among  the  more  celebrated 
cast-iron  bridges  in  England,  that  of  Coalhrookdale  belongs 
to  the  first  epoch  above  mentioned ;  those  of  Staines  and 
Sunderland  to  the  second ;  and  to  the  third,  the  bridge 
of  Soiothtuark  at  London ;  that  of  Tewkesbury  over  the 
Severn  ;  that  over  the  Lary  near  Plymouth,  and  a  number  of 
others  in  various  parts  of  the  United  Kingdom. 

The  French  engineers  have  not  only  followed  the  lead 
set  them  by  the  English,  but  have  taken  a  new  step,  in 
the  tubular-shaped  ribs  of  M.  Polonceau.  The  Pont  des 
Arts  at  Paris,  a  very  light  bridge  for  foot-passengers  only, 
and  which  is  a  combination  of  cast  and  wrought  iron,  belongs 
to  their  earliest  essays  in  this  line ;  the  Pont  d'A  usterlitZj 
also  at  Paris,  which  is  a  combination  similar  to  those  of 
Staines  and  Sunderland,  belongs  to  their  second  epoch  ;  and 
the  Pont  du  Carrousel,  in  the  same  city,  built  upon  Polon- 
ceau's  system,  with  several  others  on  the  same  plan,  belong 
to  the  last. 

In  the  United  States  a  commencement  can  hardly  be  said 
to  have  been  made  in  this  branch  of  bridge  architectiu-e ; 


CAST-IRON  BRIDGES. 


327 


the  bridge  of  eighty  feet  span,  with  tubular  ribs,  constructed 
by  Major  Delafield  at  Brownsville,  stands  almost  alone, 
and  is  a  step  contemporary  with  that  of  Polonceau  in  France. 

Tlie  following  Table  contains  a  summary  description  of 
some  of  the  most  noted  European  cast-iron  bridges  : 


NAME  OF  BRIDGE. 


Coalbrookdale . 
Wearmouth . . . 

Staines  

Aiisterlitz  

Vauxhall  

Southwark  

Tewkesbury . . . 

Lary  

Cairousel  


Eiver. 


Severn. 
Wear . . 


Seine  

Thames 
Thames 
Severn.., 

Lary  

Seine  . . . 


No.  of 

Span  in 

Rise  in 

No.  of 

Date. 

Engineer. 

arches. 

feet. 

feet. 

ribs. 

1 

100.5 

40 

5 

1779 

1 

240 

30 

6 

1796 

Burdon. 

1 

181 

16.5 

1802 

5 

100.6 

10.6 

7 

1805 

Lamande, 

9 

78 

9 

1816 

Walker. 

3 

240 

24 

8 

1818 

Rennie. 

1 

170 

17 

6 

Telford. 

5 

100 

14.5 

5 

1827 

Rendel. 

3 

150 

16 

5 

1838 

Polonceau. 

628.  Iron  Arches.  Cast-iron  arches  may  be  used  for  the 
same  objects  as  those  of  timber.  The  frames  for  these  pur- 
poses consist  of  several  parallel  ribs  of  uniform  dimensions, 
which  are  cast  into  an  arch  form,  the  ribs  being  connected 
by  horizontal  ties,  and  stiffened  by  diagonal  braces.  The 
weight  of  the  superstructure  is  transmitted  to  the  curved 
ribs  in  a  variety  of  ways ;  most  usually  by  an  open  cast- 
iron  beam,  the  lower  part  of  which  is  so  shaped  as  to  rest 
upon  the  curved  rib,  and  the  upper  part  suitably  formed  for 
the  object  in  view.  These  beams  are  also  connected  by  ties, 
and  stiffened  by  diagonal  braces. 

Each  rib,  except  for  narrow  spans,  is  composed  of  several 
pieces,  or  segments,  between  each  pair  of  which  there  is  a 


Fig.  163— Repre- 
sents a  portion 
of  a  cast-iron 
plate  arch  with 
an  open  cast-iron 
beam. 

A,  A,  segments  of 
the  arch. 

B,  B,  panels  of  the 
open  beam  con- 
nected at  the 
joints  a  b. 


328 


dVTL  ENGINEERING. 


joint  in  the  direction  of  the  radius  of  curvature.  The  forms 
and  dimensions  of  the  segments  are  uniform.  The  segments 
are  usually  either  solid  (Fig.  163)  or  open  plates  of  uniform 
thickness,  having  a  flanch  of  uniform  breadth  and  depth  at 
each  end,  and  on  the  entrados  and  intrados.  The  flanch  serves 
both  to  give  strength  to  the  segment  and  to  form  the  connection 
between  the  segments  and  the  parts  which  rest  upon  the  rib. 

The  ribs  are  connected  by  tie-plates,  which  are  inserted  be- 
tween the  joints  of  the  segments,  and  are  fastened  to  tlie  seg- 
ments by  iron  screw  bolts,  which  pass  through  the  end  flanches 
of  the  segments  and  the  tie-plate  between  them.  The  tie- 
plates  may  be  either  open  or  solid  ;  the  former  being  usually 
preferred  on  account  of  their  superior  lightness  and  cheapness. 

The  framework  of  the  ribs  is  stiffened  by  diagonal  pieces, 
which  are  connected  either  with  the  ribs  or  the  tie-plates. 
The  diagonal  braces  are  cast  in  one  piece,  the  arms  being 
ribbed,  or  feathered^  and  tapering  from  the  centre  towards 
the  ends  in  a  suitable  manner  to  g-ive  lis^htness  combined  with 

o  to 

Strength. 

The  open  beams  (Fig.  163),  which  rest  upon  the  curved  ribs, 
are  cast  in  a  suitable  number  of  panels ;  the  joint  between 
each  pair  being  either  in  the  direction  of  the  radii  of  the  arch, 
or  else  vertical.  These  pieces  are  also  cast  with  flanches,  by 
which  they  are  connected  together,  and  with  the  other  parts 
of  the  frame.  The  beams,  like  the  ribs,  are  tied  together  and 
stiffened  l)y  ties  and  diagonal  braces. 

Beams  of  suitable  forms  for  the  purposes  of  the  structure 
are  placed  either  lengthwise  or  crosswise  upon  the  open 
beams. 

629.  Curved  ribs  of  a  tubular  form  have,  within  a  few  years 
back,  been  tried  with  success,  and  bid  fair  to  supersede  the 
ordinary  plate  rib,  as  with  the  same  amount  of  metal  they 
combine  more  strength  than  the  flat  rib. 

The  application  of  tubular  ribs  was  flrst  made  in  the  United 
States  by  Major  Delafleld  of  the  U.  S.  Corps  of  Engineers,  in 
an  arch  for  a  bridge  of  SO  feet  span.  Each  rib  was  formed  of 
nine  segments ;  each  segment  (Fig.  164)  being  cast  in  one 
piece,  the  cross  section  of  which  is  an  elliptical  rhig  of  uni- 
form thickness,  the  transverse  axis  of  the  ellipse  being  in 
the  direction  of  the  radius  of  curvature  of  the  rib.  A  broad 
elliptical  flancli  with  ribs,  or  stays,  is  cast  on  each  end  of  the 
segment,  to  connect  the  parts  with  each  other ;  and  three 
chairs^  or  saddle-jpieces,  with  grooves  in  them,  are  cast  upon 
the  entrados  of  each  segment,  and  at  equal  intervals  apart,  to 
receive  the  open  beam  which  rests  on  the  curved  rib. 


CAST-IKON  BKEDGES. 


329 


The  ribs  are  connected  by  an  open  tie  plate  (Fig.  164). 
Raised  elliptical  projections  are  cast  on  each  face  of  the  tie 
plate,  where  it  is  connected  with  the  segments,  which  are 
adjusted  accurately  to  the  interior  surface  of  each  pair  of 
segments,  between  which  the  tie  plate  is  embraced.  The 
segments  and  plate  are  fastened  by  screw  bolts  passed 
through  the  end  fianches  of  the  segments. 


Fig.  164 — Represents  a  side  view  A,  and  a  cross  section  and  end  view  B,  through  a  saddle-piece 

of  the  tubular  arch  of  Major  Delafield. 
a,  a  (Fig.  A),  a  side  view,  and  (Fig.  B)  an  end  view  of  the  elliptical  flanches  of  the  end  of  each 

segment. 

6,  6,  shoulders,  or  ribs  to  strengthen  the  flanches  against  lateral  strains, 
c,  tie-plate,  between  the  ribs. 

/,  (Fig.  B)  side  view  of  the  rim  of  the  tie-plate  fitted  to  the  interior  of  the  tube. 

^)     (Figs.  A  and  B)  saddle-pieces  to  receive  the  open  beams  of  a  form  similar  to  Fig.  163, 

which  rest  on  the  tubular  ribs. 
e,  cross  section  of  the  rib  through  the  saddle-piece. 

The  tie  plates  form  the  only  coimection  between  the  curved 
ribs ;  the  broad-ribbed  fianches  of  the  segments,  and  the 
raised  rims  of  the  tie  plates  inserted  into  the  ends  of  the 
tubes,  giving  all  the  advantages  and  stiffness  of  diagonal 
pieces. 

630.  Tubular  ribs  with  an  elliptical  cross  section  have 
been  used  in  France  for  many  of  their  brida^es.  They  were 
first  introduced  but  a  few  years  back  by  M.  rolonceau,  after 


S30 


CIYIL  ENGINEEEING. 


Fig.  165. — Represents  a  side  view  A,  and  a  cross  section  and  end  view  B,  through  a  joint  of  M. 

Polonceau's  tubular  arch, 
a,  a.  top  fJanch,  6,  b.  bottom  flanch  of  the  semi-segments  united  along  the  vertical  joint  cd 

through  the  axis  of  the  rib. 
g  h,  side  view  of  the  joijit  between  the  flanches  e,  e  of  two  semi-segments. 
m,  inner  side  of  the  flanches. 

c,  cross  section  of  a  semi-segment  and  top  and  bottom  flanches. 

f,  /,  thin  wedges  of  wroiight  iron  placed  between  the  end  flanches  of  the  semi-segments  to  bring 
the  parts  to  their  proper  bearing. 

whose  designs  the  greater  part  of  these  structures  have  been 
built.  According  to  M.  Polonceau's  plan,  each  rib  consists 
of  two  symmetrical  parts  divided  lengthwise  by  a  vertical 
joint.  Each  half  of  the  rib  is  composed  of  a  number  of 
segments  so  distributed  as  to  break  joints,  in  order  that  when 
the  segments  are  put  together  there  shall  be  no  continuous 
cross  joint  through  the  ribs. 

The  segments  (Fig.  165)  are  cast  with  a  top  and  bottom 
flanch,  and  one  also  at  each  end.  The  halves  of  the  rib  are 
connected  by  bolts  through  the  npper  and  lower  flanches, 
and  the  segments  by  bolts  through  the  end  flanches. 

For  the  purposes  of  adjusting  the  segments  and  bringing 
the  rib  to  a  suitable  degree  of  tension,  flat  pieces  of  wrought 
iron  of  a  wedge  shape  are  driven  into  the  joints  between  the 
segments,  and  are  confined  in  the  joints  by  the  bolts  which 
fasten  the  segments  and  which  also  pass  through  these 
wedges. 

To  connect  the  ribs  with  each  other,  iron  tubular  pieces 
are  placed  between  them,  the  ends  of  the  tubes  being  suita- 
bly adjusted  to  the  sides  of  the  ribs.  Wrought-iron  rods 
which  serve  as  ties  pass  through  the  tubes  and  ribs,  being 
arranged  with  screws  and  nuts  to  draw  the  ribs  flrmly  against 
the  tubular  pieces.    Diagonal  pieces  of  a  suitable  form  are 


CAST-IEON  BRIDGES. 


331 


placed  between  the  ribs  to  give  tbem  the  requisite  degree  of 
stiffness. 

In  the  bridges  constructed  by  M.  Polonceau  according  to 
this  plan,  he  supports  the  longitudinal  beams  of  the  roadway 
by  cast-iron  rings  which  are  fastened  to  the  ribs  and  to 
each  other,  and  bear  a  chair  of  suitable  form  to  receive  the 
beams. 

631.  Open  cast-iron  beams  are  seldom  used  except  in  com- 
bination with  cast-iron  arches.  Those  of  wrought  iron  are 
frequently  used  in  structures.  They  may  be  formed  of  a 
top  and  bottom  rail  connected  by  diagonal  pieces,  forming  the 
ordinary  lattice  arrangement,  or  a  piece  bent  into  a  curved 


,1 


Fig.  166 — Represents  an  open  beam 
of  wrought  iron  consisting  of  a  top 
and  bottom  rail  a  and  &,  with  an 
intermediate  curved  piece,  the 
whole  secured  by  the  pieces  c,  c,  in 
pairs  bolted  to  them. 

d,  e,  and  /  represent  the  parts  of  a 
truss  of  a  curved  light  roof,  con- 
nected with  the  open  beam ;  and 
also  the  manner  in  which  the 
whole  are  secured  to  the  wall. 


form  may  be  placed  between  the  rails,  or  any  other  suitable 
combination  (Fig.  166)  may  be  used  which  combines  lightness 
with  strength  and  stiffness. 

632.  Effects  of  Temperature  on  Stone  and  Cast-iron 
Bridges.  The  action  of  variations  of  temperature  upon 
masses  of  masonry,  particularly  in  the  coping,  has  already  been 
noticed.  The  effect  of  the  same  action  upon  the  equilibrium 
of  ai-ches  was  first  observed  by  M.  Yicat  in  the  stone  bridge 
built  by  him  at  Souillac,  in  the  joints  of  which  periodical 
changes  were  found  to  take  place,  not  only  from  the  ranges 
of  temperature  between  tlie  seasons,  but  even  daily.  Similar 
phenomena  were  also  very  accurately  noted  by  Mr.  George 
Rennie  in  a  stone  bridge  at  Staines. 

From  these  recorded  observations  the  fact  is  conclusively 
established,  that  the  joints  of  stone  bridges,  both  in  the  arches 


332 


CIVIL  ENGINEEKING. 


and  spandrels,  are  periodically  affected  by  this  action,  which 
must  consequently  at  times  throw  an  increased  amount  of 
pressure  upon  the  abutments,  but  without,  under  ordinary 
circumstances,  any  danger  to  the  permanent  stability  of  the 
structure. 

When  iron  was  first  proposed  to  be  employed  for  bridges, 
objections  were  brought  against  it  on  the  ground  of  the  ejffect 
of  changes  of  temperature  upon  this  metal.  The  failure  in 
the  abutments  of  the  iron  bridge  at  Staines  was  imputed  to 
tliis  cause,  and  like  objections  were  seriously  urged  against 
other  structures  about  to  be  erected  in  England.  To  put 
this  matter  at  rest,  observations  were  very  carefully  made  by 
Sir  John  Rennie  upon  tlie  arches  of  Southwark  bridge,  built 
by  his  father.  From  these  experiments  it  appears  that  the 
mean  rise  of  the  centre  arch  at  the  crown  was  about  jqUi  of 
an  inch  for  each  degree  of  Fahr.,  or  1.25  inches  for  50°  Fahr. 
The  change  of  form  and  increase  of  pressure  arising  from 
this  cause  do  not  appear  to  have  affected  in  any  sensible 
degree  the  permanent  stability  either  of  this  structure,  or  of 
any  of  a  like  character  in  Europe. 


Y. 

IKON  TRUSSED  BRIDGES. 

633.  Among  the  earliest  and  most  meritorious  of  the  iron 
bridges  of  this  coantry  is  Whipple's  Trapezoidal  Truss  (see 
Fig.  167).  So  far  as  the  arrangement  of  ties  and  struts  are 
concerned  it  is  similar  to  the  Pratt  Truss. 


 T  T 

 TP  Tj 

I  ym  / 

f           1  / 

Fier.  1()7. — The  upper  chord  is  of  cast-iron  and  made  in  sections,  the  length  of  each  piece  be- 
ing equal  to  the  length  of  a  bay.  The  lower  chord  is  composed  of  a  succession  of  links  (see 
Fig.  1(>8),  which  receive  cast-iron  blocks  at  their  ends.  The  cast-iron  blocks  form  steps  for 
securing  the  lower  ends  of  the  vertical  posts.  The  posts  have  openings  near  the  middle  of 
their  length,  through  which  the  main  and  counter- ties  pass.  The  posts  are  trussed  at  the 
middle,  as  shown  in  the  figure. 

In  this  truss  the  end  members  are  inclined,  so  that  the 
general  form  of  the  outline  is  that  of  a  trapezoid.    All  un- 


IKON  BRIDGES. 


333 


necessary  members  are  omitted,  and  hence  comparatively  few 
comiter-ties  are  used.  In  the  Fig.  only  two  are  shown — one 
each  side  of  the  centre.  The  number  of  counter-ties  depends 
upon  the  relation  of  the  moving  load  to  that  of  the  weight  of 
the  bridge  (see  articles  107  and  108  of  "Wood's  Treatise  on 
Bridges' and  Roofs). 

The  lower  chord  is  sometimes  made  of  links  of  iron  (Fig. 
168),  which  pass  over  cast-iron  blocks  under  the  vertical 

y\  Fig  168.— One  of  the  links  of  the  lower 
 J)  chord. 


posts  (Fig.  169).    The  lower  chord  may  be,  and  at  the  pres- 


Fig  169. — A  joint  in  the  lower  chord  of  a  Whipple  Truss. 


ent  day  often  is,  made  of  eye-bars  (Fig.  170).    The  proper 


Fig.  170.— One  end  of  an  eye-bar  used  in  tension  members  of  bridges 
and  roofs. 


form  and  dimensions  of  the  eyes  and  the  proper  size  of  the 
pins  has  been  the  subject  of  considerable  experiment. 

At  first  it  was  supposed  that  the  total  section  on  both  sides 
of  the  eye  should  equal  half  the  section  of  the  pin,  but  ex- 
periments quickly  showed  that  when  made  in  this  proportion 
the  eyes  would  tear  out  before  the  shearing  strength  of  the 
pin  was  reached.  According  to  some  experiments  made  by 
Sir  Charles  Fox,  he  concluded  that  it  was  best  to  make  the 
bearing  surface  between  the  pin  and  concave  surface  of  the 
eye  about  equal  to  the  least  section  of  the  link ;  or,  in  other 
words,  the  diameter  of  the  pin  should  equal  about  two-thirds 
of  the  diameter  of  the  link. 

This  rule,  however,  is  not  rigidly  adhered  to  by  our  most 
eminent  bridge  builders.  Each  has  a  rule  of  his  own.  Some 
make  the  eye  thicker  than  the  link ;  others  make  them  some- 
what pear-shaped  by  adding  material  back  of  the  eye  (Fig. 
171) ;  while  still  others  make  them  of  the  form  shown  in  Fig. 
172. 


o 


Fig  171. — Form  of  eye-bar. 


33-1 


CIVIL  ENGINEERING. 


But  in  all  cases  the  total  section  of  the  material  through 
the  eye  is  made  to  exceed  that  through  the  bar,  and  the  sec- 
tion of  the  pin  also  exceeds  that  of  the  bar. 


I  j    Fig.  172 — Another  form  of  eye-bar. 


634.  Modifications  of  Whipple's  Truss.  Different 
bridge  builders  have  modified  the  details  of  Whipple's  Truss, 
so  as  to  suit  their  convenience  or  fancy,  or  to  make  them  con- 
form with  modern  practice.  It  is  useless  to  attempt  to  give 
all  these  modifications.  They  have,  however,  given  rise  to 
certain  names  of  bridges,  such  as  the  Murphy-Whipple 
bridge,  Linville  bridge,  Jones's  bridge,  etc.,  etc. 

635.  Linville  Bridge.  This  bridge,  the  details  of  which 
(Figs.  173  and  174)  have  been  very  thoroughly  and  carefully 
worked  out,  has  a  wide  reputation. 

The  improvements  consist  in  employing  tubular  forms  of 
wrought  iron  for  members  used  to  resist  compressive  strains, 
and  weldless  eye-bars  to  resist  tensile  strains,  by  this  means 
economizing  material  and  reducing  the  dead  weight  of  the 
structures.  In  the  accompanying  details  of  the  chords,  struts, 
and  ties,  and  the  floor  system  and  lateral  connections,  some  of 
the  leading  principles  of  the  Linville  truss  are  illustrated. 

The  upper  chords  A,  are  composed  of  channel  (  [)  bars 
and  I  beams,  to  which  are  riveted  top  plates,  and  sometimes 
bottom  plates,  forming  a  tubular  compressive  member  of 
great  strength.  When  the  lower  plate  is  used,  elliptical  holes 
are  cut  out  in  order  to  admit  of  painting  the  interior.  The 
chords  are  generally  made  in  sections,  one  panel  in  length. 
The  connection  between  the  suspension  ties  and  upper  chords 
are  effected  by  means  of  angle  blocks,  through  which  pass 
the  suspension  ties,  with  enlarged  screw  threads  and  nuts  for 
adjustment,  or  by  means  of  pins  passing  through  the  chords, 
and  through  loops  or  eyes  on  the  suspension  ties. 

The  struts  B  are  circular  or  polygonal  tubes  (Fig.  174(^), 
composed  of  two  or  more  rolled  bars  united  by  rivets  througli 
flanges,  or  by  transverse  tie-bolts  passing  through  the  struts 
between  the  flanges.  The  struts  are  generally  swelled  and 
opened  to  allow  the  interior  to  be  repainted  in  order  to  -pre- 
vent  their  rapid  destruction  by  oxydation. 

The  lower  chords  are  made  by  upsetting  the  enlarged  eye 
ends,  by  compressing  them  when  highly  heated  into  moulds 
or  dies.  They  are  afterwards  forged  and  rewelded  under  a 
hammer. 


IRON  BRroGES. 


335 


Figs.  173,  174 — Details  of  Linville's  txuss.  Fig.  173  is  a  cross  section,  and  Fig.  174  a  right 
section  of  a  portion  of  the  truss. 

A  A,  upper  chord,  composed  of  channel  bars  (  [  )  and  I  sections.  B,  the  post.  (See  Fig. 
174  A.)  'CO,  the  lower  chord.  D  D,  the  lower  end  of  a  main  tie  ;  and  H  H,  the  upper  end  of 
a  main  tie.  E  is  a  counter-tie.  G  G-,  bases  of  the  posts  or  struts. 

1 1,  suspenders  for  supporting  cross-ties.       ,  J,  cross  horizontal  diagonal  tie. 

K,  horizontal  diagonal  tie, 

L 


336 


CIVIL  ENGLNEERING. 


A— Cross  section  of  one  of  the  forms  of  post  used  in  a  Lin- 


Tliese  weldless  chords  and  tubular  posts  have,  in  many 
cases,  superseded  older  foi-ms.  The  lower  chords  C  C  disposed 
at  each  side  of  the  suspension  ties  D,  and  counter-tie  E,  and 
between  ribs  in  the  bases  G  of  the  posts  or  struts,  are  effect- 
ually combined  with  the  struts  and  ties  by  means  of  a  con- 
necting-pin. The  tendency  to  bend  the  connecting-pin  is 
obviated  by  this  distribution  of  the  strains. 

The  pin  can  fail  only  by  shearing. 

From  the  connecting-pins  depend  loops  or  suspenders,  1 1, 
whicli  support  the  rolled  cross-girders  F,  that  sustain  the  track- 
stringers  and  track.  The  upper  lateral  struts  of  wrought  or 
cast  iron  are  secured  at  the  connecting-pins,  the  ties  being 
attaclied  to  an  eye-plate,  or  in  a  jaw-nut  secured  to  the  con- 
necting-pins. 

The  lateral  ties  J  are  adjusted  by  means  of  sleeve-nuts  with 
right  and  left  hand-screws. 

The  lower  laterals  K  K  are  attached  to  the  cross  girders, 
and  adjusted  in  a  similar  manner. 

The  bases  and  capitals  of  the  posts  are  made  either  of 
wrought  or  cast  iron. 

To  secure  greater  efficiency  in  the  struts  by  dispensing 
with  the  round  bearing,  and  at  the  same  time  retain  the  pin 


Fig.  175— An  Arched  Tru!?s  after  the  general  plan  of  Whipple's.  The  lower  chords  or  tie- 
rods  pass  through  the  ends  of  the  arch,  and  are  secured  by  nuts  on  the  ends  of  the  rods. 


lEON  BRIDGES. 


337 


connection  between  the  chords  and  ties,  the  lower  chords 
are  brought  compactly  together  between  and  outside  of  the 
suspension-ties  and  suspenders,  and  a  bearing  provided  on  the 
upper  edges  of  the  chords  for  the  lower  ends  of  the  posts. 
The  upper  ends  also  have  a  flat  bearing. 

636.  Arched  Truss.  Fig.  175  shows  the  general  form 
of  a  Whipple  Arched  Truss.  The  upper  chord  is  composed 
of  hollow  tubes,  made  in  sections  of  about  a  panel  length. 

637.  Bollman's  Truss.  The  general  outline  of  Bollman's 
Truss  is  shown  in  Fig.  176. 


C  F  C  J  L 


Fig  176 — ^Bollman'B  Truss,  A  D,  D  E,  etc.,  are  sections  of  the  uppper  chord — cast  iron  and 
usually  hollow.  D  C,  E  F,  etc.,  are  hollow  cast-iron  posts.  AC,  C  B ;  A  F,  F  B,  etc.,  are 
tension  rods ;  D  F,  C  E,  etc.,  are  panel  rods. 

One  of  the  leading  features  of  this  bridge  is,  the  load  at 
each  post  or  joint  is  carried  directly  to  the  supports  at  the 
ends  by  means  of  a  pair  of  tension  (or  suspension)  rods. 
Thus  a  load  at  E  is  supported  by  the  post  E  F,  and  is  thence 
supported  by  the  rods  A  F  and  F  B.  The  panel  rods  D  F, 
E  C,  E  G,  etc.,  serve  to  keep  the  upper  chord  in  place,  and 
in  case  of  an  undue  strain  upon,  or  failure  of,  one  of  the  long 
suspension-rods,  may  transmit  the  strains  to  the  other  mem- 
bers of  the  truss. 

The  suspension  rods  being  of  unequal  length  will  be 
unequally  elongated  or  contracted  by  the  same  strain,  or 
by  changes  in  the  temperature.  In  order  to  prevent  severe 
cross  strains  upon  the  posts  due  to  these  causes,  the  suspension- 
rods  are  connected  to  the  lower  ends  of  the  posts  by  means 
of  a  link  which  is  a  few  inches  in  length,  and  which  permits 
of  a  small  lateral  movement  at  the  ends  of  the  rods  without 
any  corresponding  movement  of  the  posts.  The  suspension- 
rods  are  made  of  flat  iron,  and  pass  through  the  ends  of  the 
upper  chord  where  they  are  secured  by  means  of  pins  which 
pass  through  the  ends  of  the  chords. 

If  the  roadway  passes  above  the  upper  chord,  it  is  called  a 
deck  bridge,  and  the  lower  chord  may  be  dispensed  with. 
22 


338 


CIVIL  ENGmEERING. 


But  if  it  passes  on  the  level  of  the  lower  chord  (Fig.  17&a.) 

the  lower  chord  may  be  simply  suspended  upon  the  posts  ; 
and  not  be  depended  upon  for  resisting  tension.  The  lower 
chord  in  this  case  may  also  be  entirely  dispensed  with ;  for 
cross-ties,  or  joists,  may  be  secured  to  the  posts  and  longi- 
tudinal joints  be  placed  upon  them.  If  the  lower  chord  is 
used  and  is  made  continuous  so  as  to  resist  tension,  it  vir- 


Pig.  176a. 


tually  changes  it  into  a  Whipple  truss  in  which  the  long  sus- 
pension-rods are  unnecessary  members.  Still,  in  this  case, 
the  truss — especially  the  panel  rods,  are  not  so  proportioned 
as  to  make  it  safe  to  omit  the  long  suspension-rods. 

638.  Fink  Truss.  The  outline  or  skeleton  of  a  Fink  truss 
is  shown  in  Fig.  177. 


A 


a       J)       c        D       d      ^      y  \ 

fx 

V 

3     A      I      ^     J     Jc  / 


Fig.  177— Fink  Trass.  A  B  the  upper  chord,  g  I  the  lower  chord,  a  g,hfi,  etc.,  are  posts. 
A  C,  C  B  long  suspension-rods.   A  A,  A  D,  etc.,  secondary  suspension-rods. 

This  truss  consists  of  a  primary  system  of  king  posts,  A  C 
B,  Fig.  177 ;  two  secondary  systems,  A  A  D  and  1)  ^  B ;  four 
tertiary  systems,  A  g  h,  b  i  I),!)  j  e,  and  e  IB,  and  so  on. 


IRON  BRIDGES. 


339 


The  posts,  suspension-rods  and  chords  may  be  similar  in 
detail  to  the  systems  previously  described. 

The  noted  Louisville  bridge,  across  the  Ohio  Biver  at 
Louisville,  is  made  upon  this  plan. 


DIMENSIONS  OF  THE  LOUISVILLE  BRIDGE. 

It  is  5,294  feet  long,  divided  into  the  following  spans  from 
centre  to  centre  of  piers : 


span 


32.5  feet. 

100.0 

a 

264.0 

u 

598.4 

(6 

360.0 

u 

420.0 

454.0 

a 

3T0.0 

C6 

1,473.0 

400.0 

(C 

540.0 

6i 

149.6 

cc 

100.0 

(( 

32.5 

a 

5,294.0 

(( 

Total  length   5,294.0 

639.  Post's  Truss.  The  main  peculiarity  of  this  truss  is 
in  its  form.  The  upper  ends  of  the  posts  are  carried  towards 
the  centre  of  the  bridge,  an  amount  equal  to  half  a  bay,  and 
as  all  the  bays  are  equal  the  posts  in  each  half  of  the  truss  are 
all  parallel  to  each  other  (Fig.  178). 


T^g.  178— Side  view  of  panels  of  a  Post  Truss.  A  A  are  struts.  B  B,  main  ties.  C  C,  counter 
ties.  E  E,  bottom  chords.  I  I,  top  chords.  D,  ends  of  floor-beams.  G,  lower  horizontal 
diagonal  ties.   G',  upper  horizontal  diagonal  ties. 


340 


CmL  ENGINEERING. 


Tig.  180— Plan  of  the  top  of  the  bridge.  I,  top  chord.  H,  cross-tie  or  strut.  Q\  upper  hori- 
zontal tie. 


"Fig.  181— Shows  details  at  a  joint  of  the  lower  chord.  F  is  a  cast-iron  block  for  receiving 
the  ends  of  the  horizontal  tie-rods.  K  is  an  iron  bolt  which  passes  through  the  ends  of  the 
links  which  form  the  lower  chord.  The  other  letters  refer  to  the  same  parts  as  in  the  preced- 
ing figures. 

DESCRIPTION  OF  POSt's  IRON  BRIDGE. 

A  A  (Figs.  178,  179,  180  and  181)— Are  the  struts,  com- 
posed of  two  rolled-iron  channel  bars,  with  plates  riveted  on 
their  flanores,  formino:  a  hollow  column  havinor  a  rectang:ular 
cross-section.    The  struts  are  swelled  in  the  centre  by  spring- 


IKON  BRIDGES. 


341 


ing  the  channel-bars  and  having  the  plates  sheared  to  the 
required  shape. 

The  bearings  of  the  struts  upon  the  pins  (K)  are  of  either 
cast  or  wrought  iron,  and  are  enclosed  between  the  side-plates, 
and  abut  against  the  channel-bars,  and  are  riveted  to  both. 
The  pin  holes  are  bored  through  shoes  and  plates. 

B  JB — Are  the  main  ties,  or  main  suspension  braces,  and 
are  made  of  flat  bar-iron  with  die-forged  heads  at  the  ends, 
bored  out  to  fit  the  pins. 

C  C — Are  the  counter  ties,  made  of  round  iron,  with 
forged  eyes  at  the  ends  to  receive  the  pins,  and  having  turn- 
buckles  at  a  convenient  distance  from  the  bottom  end,  for 
purposes  of  adjustment. 

D  D — Are  the  floor-beams,  suspended  in  pairs  from  the 
chord  pins  at  each  panel  point,  by  means  of  eye-bolts  or  by 
stirrups  passing  over  the  chord  pins  and  under  a  bolt  through 
the  webs  of  the  beams. 

E  E — Are  the  bottom  chord  bars  or  links,  made  of  flat  bar- 
iron,  with  die -forged  heads,  and  bored  holes  for  the  chord 
pins.  The  sizes  of  the  bars  in  the  respective  panels  are  de- 
termined by  the  strains,  the  first  and  second  panels  having 
two  bars,  the  third  and  fourth  having  four  bars  each,  the  fifth 
and  sixth  having  six  bars  each,  etc.,  to  the  centre  of  the  span. 

F — Is  a  bottom  lateral  brace  angle  block  of  cast  iron,  fas- 
tened to  the  ends  of  the  floor-beams,  which  form  the  bottom 
lateral  strut. 

G  G — Are  the  lateral  brace-rods,  of  round  iron,  having 
screws  and  nuts  at  their  ends,  for  adjustment. 

H  H — Are  top  lateral  struts,  made  of  rolled-iron  |  beams, 
or  channel  bars  in  pairs.  These  struts  have  a  cast-iron  shoe 
at  their  ends,  and  are  bolted  to  the  top  plate  of  the  top  chord, 
by  bolts  passing  through  shoes,  top  plate  of  chord,  and  through 
the  joint  box  in  the  top  chord.  The  top  lateral  brace  rods 
pass  through  the  cast-iron  shoes,  with  nuts  on  the  outside. 

I  I — Are  the  top  chords.  When  made  of  wrought  iron 
they  are  composed  of  channel  bars  with  covering  plate  rivet- 
ed to  the  flanges  on  the  top,  and  bars  riveted  at  intervals 
across  the  bottom  flanges,  either  diagonally  or  straight  across 
to  keep  the  channel  bars  in  line.  Additional  sectional  area  is 
obtained  by  riveting  plates  on  the  inside  of  the  channel  bars. 

The  top  chords  are  made  in  panel  lengths,  with  their  ends 
squared  by  machinery  to  insure  true  bearings — and  when  of 
cast  iron  have  a  rectangular  cross-section,  with  the  inside 
cored  out  to  obtain  the  necessary  sectional  area  to  provide  for 
the  compression  strain. 


34:2 


CIVIL  ENGINEERING. 


The  connection  of  the  stmts  and  main  and  counter  braces 
is  made  by  means  of  a  pin  passing  through  a  cast-iron  box 
which  encloses  the  mall,  the  length  of  the  pin  being  just  equal 
to  the  width  of  the  box.  The  top-chord  sections  have  a  recess 
which  fits  over  the  box,  and  when  the  connection  is  made  in 
the  box  the  pieces  of  top  chord  are  laid  on,  and  cover  the 
whole.  The  joint  is  then  secured  by  the  bolts  which  pass 
through  the  top  lateral  strut,  top  chord  and  joint  box. 

DESCRIPTION  OF  POSt's  "  COIVEBINATION  "  OR  "  COMPOSITE  "  BRIDGE. 

This  bridge  is  composed  partly  of  wood  and  partly  of  iron, 
as  shown  in  Figs.  181^^,  181^,  and  181c. 

A,  A — Top  chord,  packed  and  framed  as  shown  in  Figs. 
181^^  and  1815. 

B  B — Struts,  framed  with  square  end  at  the  top  entering  and 
abutting  against  joint  box  E  (Fig.  1815)  and  fitted  at  bottom 
ends  into  strut  shoe  K  (Fig.  181o). 


Fig.  181a. 

C  C — Main  suspension  ties,  of  square,  round  or  flat  iron, 
with  eye  at  lower  end  and  screw  at  upper  end,  passing  through 
joint  box  E,  secured  by  nuts. 

D  D — Counter  braces,  of  square  or  round  iron,  made  sim- 
ilar to  main  ties. 

E  E — Cast-iron  joint  boxes  enclosed  in  top  chord,  and 
receiving  the  struts,  main  ties  and  coimters. 

This  box  has  a  flange  around  the  bottom  to  support  the 
weight  of  the  top  chord,  which  lies  upon  and  is  bolted  to  it. 


ERON  BRIDGES. 


343 


F  F-— Bottom  chord  links  of  flat  iron,  with  heads  at  each 
end,  bored  to  receive  the  pins  (Fig.  181c). 


Fig.  181c. 


G  G — Kolled  iron  floor-beams,  suspended  to  chord  pins. 
H  H — Bottom  lateral  ties,  roimd  iron  rods  with  screws. 
1 1 — Bottom  lateral  angle  block,  cast  iron. 
K — Cast-iron  strut  shoes,  having  sockets  to  receive  struts 


CIVIL  ENGINEERING. 


and  drilled  holes  for  chord  pins  passing  through  flanges  or 
ribs  below  the  sockets. 

640.  Alleghany  River  Bridge  at  Pittsburgh,  Pa.  This 
is  a  lattice  iron  bridge  (Fig.  182),  and  is  similar  to  several 


Fig.  182.  * 


other  structures  which  have  been  made  in  this  country.  There 
is  a  similar  one  on  the  New  York  Central  Railroad,  at  Sche- 
nectady,     Y.,  and  another  near  Rome,  of  the  same  State. 

641.  St.  Louis  and  Illinois  Bridge.  This  noted  structure 
might  properly  be  called  a  steel  arch.  It  is  now  in  course  of 
erection,  and  is  to  consist  of  three  spans,  the  central  one  of 
which  is  615  feet,  and  each  of  the  end  ones  497  feet.  There 
are  eight  arches  in  each  span,  arranged  in  sets  of  two  and 
two ;  and  in  each  set  one  arch  is  directly  over  the  other,  and 
the  two  are  trussed  together  by  link-bars.  The  arches  are 
composed  of  steel  tubes,  which  are  made  of  steel  staves,  as 
will  now  be  explained. 

All  the  steel  in  this  structure  is  of  the  very  best  quality. 
The  standard  fixed  for  it  by  the  Chief  Engineer,  Capt.  Eads, 
was  so  high  as  to  make  it  almost  impossible  for  our  best  steel 
manufacturers  to  produce  it.    The  coefiicient  of  elasticity  was 


IRON  BEIDGES. 


845 


Tig.  183 — Section  of  a  tube,  St.  Louis  and  Illinois  Bridge,  a  a  is  a  steel  caseing  abont 
three-eighths  of  an  inch  thick,  which  is  lapped  over,  and  riveted  like  the  plates  of  a  steam-boiler, 
b,  b  are  steel  staves  which  are  forced  into  the  caseing. 

A  A,  Figs.  183  and  184,  are  cross-rods  for  connecting  the  arches  together  laterally. 

B  B  B  are  diagonal  rods  in  a  vertical,  for  connecting  the  upper  arch  in  one  set  to  the  lower 
arch  in  the  adjacent  set. 

C  C  C  are  diagonal  rods  in  the  plane  of  the  tubes,  for  connecting  the  joint  of  one  set  with 
the  joint  which  is  in  advance  of  or  back  of  the  corresponding  joint  ia  the  adjacent  set. 

D  is  a  vertical  diagonal  rod  for  trussing  the  roadway. 

E  E  are  trussed  vertical  posts,  the  lower  ends  of  which  are  secured  to  the  arch,  and  the 
tipper  ends  support  the  roadway. 


"Fig.  184 — Is  a  cross-section  of  two  arches 
of  the  bridge.  Two  sets  of  tubes  which 
form  the  arch  are  shown,  also  the  posta 
and  rods  which  have  been  described  above. 

F  P  is  the  track  for  a  railroad ;  the  car- 
riage road  passing  above  this. 


346 


CIVIL  ENGINEEEING. 


to  be  between  26,000,000  pounds  and  30,000,000  pounds,  and 
it  was  to  sustain  a  strain  of  60,000  pounds,  without  producing 
a  permanent  set. 

All  the  workmanship  is  of  a  higher  order  than  is  usual  in 
bridge  construction.  Special  machines  and  tools  were  made 
for  making  the  several  joints.  An  error  of  one  thirty-second 
of  an  inch  might,  in  most  cases,  be  very  troublesome,  if  not 
fatal. 


• 

c 

o        o       o  o 

Fig.  185— Shows  a  side  view  of  a  portion  cf 
the  arch, 

Gr  G  are  diagonal  posts  which  are  trussed, 
as  shown  in  Fig.  183,  for  connecting  the  two 
arches  together.  The  other  letters  refer  to 
the  same  parts  as  in  Figs.  183  and  184. 


Fig.  186 — Shows  a  cross-Bectien  of  a  por- 
tion of  the  upper  roadway. 
1 1  is  the  carriage-way. 
H  is  the  side- walk. 


The  tubes  are  straight  throughout  their  length,  but  the 
ends  are  planed  off  in  the  direction  of  the  radius  of  the  arch, 
so  that  the  arch  is  really  a  polygon  ha\dng  short  sides.  Seve- 
ral rectangular  annular  grooves  are  cut  near  the  ends  of  each 
tube ;  and  after  the  tubes  are  put  in  place,  their  ends  abut- 
ting against  each  other,  they  are  joined,  and  firmly  secured 
by  means  of  a  heavy  and  nicely-fitted  iron  coupling.  In 
this  way  the  arch  is  made  continuous.    A  strong  steel  pin 


TUBULAR  BRIDGES. 


347 


asses  through  the  coupling  and  the  ends  of  the  tubes,  one 
alf  of  the  pin  being  in  each  tube.  One  length  of  tube  is 
put  up  at  a  time,  and  is  connected  to  all  the  others,  which 
are  properly  placed  by  cross-rods,  A  A,  Figs.  183  and  184, 
and  also  diagonal  rods  C  C  and  B  B.  The  diagonals  G  Gl- 
are also  secured.  These  are  secured  to  the  pins  c  c,  Fig.  185. 
The  vertical  posts  E  E,  which  support  the  railroad,  are  trussed 
by  means  of  diagonal  bars,  as  shown  in  Fig.  184.  Each  skew- 
back  of  the  arch  is  secured  to  the  abutments  by  means  of  two 
six-inch  steel  rods  or  bolts,  which  pass  through  the  wrought- 
iron  skew-backs,  and  several  feet  into  the  masonry.  This 
bridge,  when  completed,  will  be  one  of  the  most  remarkable 
structures  of  its  kind  in  the  world,  and  can  hardly  fail  to  es- 
tablish many  important  principles  in  iron  structures. 

642.  Kuilenberg  Bridge.  The  span  of  this  bridge  is  about 
the  same  as  that  of  the  St.  Louis  and  Illinois  bridge,  as  will 
be  seen  from  the  following  dimensions.  The  lower  chord  of 
this  bridge  (Fig.  187)  is  horizontal,  and  the  upper  chord  is 


Fig.  187— Kuilenberg  Bridge.  Span  between  the  abutments,  152  meters.  Total  length,  in- 
cluding the  parts  on  the  abutments,  156.8  meters  (about  515  feet).  Length  of  each  bay,  4 
meters.   Depth  of  the  truss  at  the  centre,  29  meters. 


the  arc  of  a  circle,  the  radius  of  which  is  809  feet.  It  is  of 
the  general  plan  of  the  Pratt  or  Whipple  systems,  only  that 
the  upper  chord  is  curved. 


YI. 


TUBULAR  BRIDGES. 


643.  Tubular  Frames  of  Wrought-iron.  Except  for  the 
obvious  application  to  steam  boilers,  sheet  iron  had  not  been 
considered  as  suitable  for  structures  demanding  great  strength, 
from  its  apparent  deficiency  in  rigidity;  and  although  the 
principle  of  gaining  strength  by  a  proper  distribution  of  the 
material,  and  of  giving  any  desirable  rigidity  by  combinations 
adapted  to  the  object  in  view,  were  at  every  moment  acted 
upon,  from  the  ever-increasing  demands  of  the  art,  engineers 
seem  not  to  have  looked  upon  sheet  iron  as  suited  to  such 


348 


CIVIL  ENGINEEEINa. 


purposes,  until  an  extraordinary  case  occurred  which  seemed 
about  to  baffle  all  the  means  hitherto  employed.  The  occa- 
sion arose  when  it  became  a  question  to  throw  a  bridge  of 
rigid  material,  for  a  railroad,  across  the  Menai  Straits ;  sus- 
pension systems,  from  their  flexibility,  and  some  actual  fail- 
ures, being,  in  the  opinion  of  the  ablest  European  engineers, 
unsuitable  for  this  kind  of  communication. 

Robert  Stephenson,  who  for  some  years  held  the  highest 
rank  among  English  engineers,  appears,  from  undisputed  tes- 
timony, to  have  been  the  first  to  entertain  the  novel  and  bold 
idea  of  spanning  the  Straits  by  a  tube  of  sheet  iron,  supported 
on  piers,  of  sufficient  dimensions  for  the  passage  within  it  of 
the  usual  trains  of  railroads.  The  preliminary  experiments 
for  testing  the  practicability  of  this  conception,  and  the  work- 
ing out  ot  the  details  of  its  execution,  were  left  chiefly  in  the 
hands  of  Mr.  William  Fairbairn,  to  whom  the  profession  owes 
many  valuable  papers  and  facts  on  professional  topics.  This 
gentleman,  who,  to  a  thorough  acquaintance  with  the  mode 
of  conducting  such  experiments,  united  great  zeal  and  judg- 
ment, carried  through  the  task  committed  to  him ;  proceed- 
ing step  by  step,  until  conviction  so  firm  took  the  place  of 
apprehension,  that  he  rejected  all  suggestions  for  the  use  of 
any  auxiliary  means,  and  urged,  from  his  crowning  experi- 
ment, reliance  upon  the  tube  alone  as  equal  to  the  end  to  be 
attained. 

Numerous  experiments  were  made  by  him  upon  tubes  of 
circular,  elliptical,  and  rectangular  cross-section.  The  object 
chiefly  kept  in  view  in  these  experiments  was  to  determine 
the  form  of  cross-section  which,  when  the  tube  was  submitted 
to  a  cross  strain,  would  present  an  equality  of  resistance  in  the 
parts  brought  into  compression  and  extension.  It  was  shown, 
at  an  early  stage  of  the  operations,  that  the  circular  and  ellip- 
tical forms  were  too  weak  in  the  parts  submitted  to  compres- 
sion, but  that  the  elliptical  was  the  stronger  of  the  two ;  and 
that,  whatever  form  might  be  adopted,  extraordinary  means 
would  be  requisite  to  prevent  the  parts  submitted  to  compres- 
sion from  yielding,  by  ^'puckering"  and  doubling.  To  meet 
this  last  difficulty,  the  fortunate  expedient  was  hit  upon  of 
making  the  part  of  the  main  tube,  upon  which  the  strain  of 
compression  was  brought,  of  a  series  of  smaller  tubes,  or  cells 
of  a  curved  or  a  rectangular  cross-section.  The  latter  form  of 
section  was  adopted  definitively  for  the  main  tube,  as  having 
yielded  the  most  satisfactory  results  as  to  resistance ;  and  also 
for  the  smaller  tubes,  or  cells,  as  most  easy  of  construction 
and  repair. 


TTEBULAE  BRmGES. 


349 


As  a  detail  of  each  of  these  experiments  would  occupy 
more  space  than  can  be  given  in  this  work,  that  alone  of  the 
tube  which  gave  results  that  led  to  the  forms  and  dimensions 
adopted  for  the  tubular  bridges  subsequently  constructed,  will 
be  given  in  this  place. 

644.  Model  Tube.  The  total  length  of  the  tube  was  78 
feet.  The  distance,  or  bearing  between  the  points  of  support 
on  which  it  was  placed  to  test  its  strength,  was  75  ft.  Total 
depth  of  the  tube  at  the  middle,  4  ft.  6^  in.  Depth  at  each 
extremity,  4  ft.    Breadth,  2  ft.  8  in. 

The  top  of  the  tube  w^as  composed  of  a  top  and  bottom 
plate,  formed  of  pieces  of  sheet  iron,  abutting  end  to  end,  and 
connected  by  narrow  strips  riveted  to  them  over  the  joints. 
These  plates  were  2  ft.  11^  in.  wide.  They  were  6^  in.  apart, 
and  connected  by  two  vertical  side  plates  and  five  interior 
division  plates,  with  which  they  were  strongly  joined  by 
angle  irons,  riveted  to  the  division  plates,  and  to  the  top  and 
bottom  plates  where  they  joined.  Each  cell,  between  two  di- 
vision plates  and  the  top  and  bottom  plates,  was  nearly  6  in. 
wide.  The  sides  of  the  tube  were  made  of  plates  of  sheet 
iron  similarly  connected;  their  depth  was  3  ft.  6f  in.  A 
strip  of  angle  iron,  bent  to  a  curved  shape,  and  running  from 
the  bottom  of  each  end  of  the  tube  to  the  top  just  below  the 
cellular  part,  was  riveted  to  each  side  to  give  it  stiffness.  Be- 
sides this,  precautions  were  finally  taken  to  stiffen  the  tube 
by  diagonal  braces  within  it.  The  bottom  of  the  tube  was 
formed  of  sheets,  abutting  end  to  end,  and  secured  to  each 
other  like  the  top  plates ;  a  continuous  joint,  running  the  en- 
tire length  of  the  tube  along  the  centre  line  of  the  bottom, 
was  secured  by  a  continuous  strip  of  iron  on  the  under  side, 
riveted  to  the  plates  on  each  side  of  the  joint.  The  entire 
width  of  the  bottom  was  2  ft.  11  in. 

The  sheet  iron  composing  the  top  cellular  portion  was  0.147 
in.  thick ;  that  of  the  sides  0.099  in.  thick.  The  bottom  of  the 
tube  at  the  final  experiments,  to  a  distance  of  20  ft.  on  each 
side  of  the  centre,  was  composed  of  two  thicknesses  of  sheet 
iron,  each  0.25  in.  thick,  the  joints  being  secured  by  strips 
above  and  below  them,  riveted  to  the  sheets ;  the  remainder, 
to  the  end  of  the  tube,  was  formed  of  sheets  0.156  in.  thick. 

The  total  area  of  sheets  composing  the  top  cellular  portion 
was  24.024  in.,  that  of  the  bottom  plates  at  the  centre  portion, 
22.450  in. 

The  general  dimensions  of  the  tube  were  one  sixth  those  of 
the  proposed  structure.  Its  weight  at  the  final  experiment, 
13,020  lbs. 


350 


CIVIL  ENGINEEEmQ. 


The  experiments,  as  already  stated,  were  conducted  with  a 
view  to  obtain  an  equality  between  the  resistances  of  the  parts 
Btrained  by  compression  and  those  extended ;  with  this  object, 
at  the  end  of  each  experiment,  the  parts  torn  asunder  at  the 
bottom  were  replaced  by  additional  pieces  of  increased 
Btrength. 

The  following  table  exhibits  the  results  of  the  final  experi- 
ments : — 


No.  of  Experiments.  Weight  in  lbs.  Deflection  in  inches. 

1   20,006  0.55 

2   35,776  0.78 

3   48,878  1.12 

4   62,274  1.48 

5   77,534  1.78 

6   92,299  2.12 

7  103,350  2.38 

8  114,660  2.70 

9  132,209  3.05 

10  138,060  3.23 

11  143,742  3.40 

12..  148,443  3.58 

13  153,027.....  3.70 

14  157,728  3.78 

15  161,886  3.88 

16  164,741  3.98 

17  167,614  4.10 

18  171,144  4.23 


19  173,912  4.33 

20  177,088  -  4.47 

21  180,017  4.55 

22.  183,779  4.62 

23  186,477  4.72 

24  189,170  4.81 

25  192,892 


The  tube  broke  with  the  weight  in  the  25th  experiment ; 
the  cellular  top  yielding  by  puckering  at  about  2  feet  from 
the  point  where  the  weight  was  applied.  The  bottom  and 
Bides  remained  uninjured. 

The  ultimate  deflection  was  4.89  in. 

645.  Britannia  Tubular  Bridge.  Nothing  further  than  a 
succinct  description  of  this  marvel  of  engineering  will  be 
attempted  here,  and  only  with  a  view  of  showing  the  arrange- 
ment of  the  parts  for  the  attainment  of  the  proposed  end. 


TUBULAR  BEIDGES. 


851 


It  differs  in  its  general  structure  from  the  model  tube,  chiefly 
in  having  the  bottom  formed  like  the  top,  of  rectangular  cells, 
and  in  the  means  taken  for  giving  stiffness  to  the  sides. 

The  total  distance  spanned  by  the  bridge  is  1,489  ft.  This 
is  divided  into  four  bays,  the  two  in  the  centre  being  each 
460  ft.,  and  the  one  at  each  end  230  ft.  each. 

The  tube  is  1,524  ft.  long.  Its  bearing  on  the  centre  pier 
is  45  ft. ;  that  on  the  two  intermediate  32  ft. ;  and  that  on 
each  abutment  17  ft.  6  in.  The  height  of  the  tube  at  the 
centre  pier  is  30  ft. ;  at  the  intermediate  piers  27  ft. ;  and  at 
the  ends  23  ft.  This  gives  to  the  top  of  the  tube  the  shape 
of  a  parabolic  curve. 


Fig.  188— Represents  a  vertical  cross-section  of  the  Britannia  Bridge. 

A,  interior  of  bridge. 

B,  cells  of  top  cellular  beam. 

C,  cells  of  bottom  cellular  beam. 

a,  top  plates  of  top  and  bottom  beams. 
6,  bottom  plates  of  top  and  bottom  beams. 

c,  division  plates  of  top  and  bottom  beams.  h 
d  and  e,  strips  riveted  over  the  joints  of  top  and  bottom  plates, 

0,  angle  irons  riveted  to  a,  6,  and  c. 
j7,  plates  of  sides  of  the  tube  A. 

h,  exterior  f  irons  riveted  over  vertical  joints  of  g, 

1,  interior  T  irons  riveted  over  vertical  joints  of  fir,  and  bent  at  the  angles  of     and  extend- 

ing beyond  the  se'cond  cell  of  the  top  beam,  and  beyond  the  first  of  the  bottom  beam, 
triangular  pieces  on  each  side  of  i,  and  riveted  to  them. 


352 


CIVIL  ENGINEERING-. 


The  cellular  top  (Fig.  188)  is  divided  into  eight  cells  by 
division  plates  connected  with  the  top  and  bottom  b,  by 
angle  irons  o,  riveted  to  the  plates  connected.  The  different 
sheets  composing  the  plates  a  and  h  abut  end  to  end  length- 
wise the  tube  ;  and  the  joints  are  secured  by  the  strips  d  and 
e,  riveted  to  the  sheets  by  rivets  that  pass  through  the  interior 
angle  irons. 

The  sheets  of  which  this  portion  is  composed  are  each  6  ft. 
long,  and  1  ft.  9  in.  wide ;  those  at  the  centre  of  the  tube  are 
^ths  of  an  inch  thick :  they  decrease  in  thickness  towards 
the  piers,  where  they  are  ths  of  an  inch  thick.  The  division 
plates  are  of  the  same  thickness  at  the  centre,  and  decrease 
in  the  same  manner  towards  the  piers.  The  rivets  are  1  inch 
thick,  and  are  placed  3  in.  apart  from  centre  to  centre.  The 
cells  are  1  ft.  9  in.  by  1  ft.  9  in.,  so  as  to  admit  a  man  for 
painting  and  repairs. 

The  cellular  bottom  is  divided  into  six  cells  C,  each  of 
which  is  2  ft.  4  in.  wide  by  1  ft.  9  in.  in  height.  To  diminish, 
as  far  as  practicable,  the  number  of  joints,  the  sheets  for  the 
sides  of  the  cells  were  made  12  ft.  long.  To  give  sufficient 
strength  to  resist  the  great  tensile  strain,  the  top  and  bottom 
plates  of  this  part  are  composed  of  two  thicknesses  of  sheet 
iron,  the  one  layer  breaking  joint  with  the  other.  The  joints 
over  the  division  plates  are  secured  by  angle  irons  o,  in  the 
same  manner  as  in  the  cellular  top.  The  joints  between  the 
sheets  are  secured  by  sheets  2  ft.  8  in.  long  placed  over  them, 
which  are  fastened  by  rivets  that  pass  through  the  triple 
thickness  of  sheets  at  these  points.  The  rivets,  for  attaining 
greater  strength  at  these  points,  are  in  lines  lengthwise  of  the 
cell.  The  sheets  forming  the  top  and  bottom  plates  of  the 
cells  are  y^^hs  of  an  inch  at  the  centre  of  the  tube,  and  de- 
crease to  xV^^^  ends.  The  division  plates  are  y^g^ths  in 
the  middle,  and  ^ths  at  the  ends  of  the  tube.  The  rivets  of 
the  top  and  bottom  plates  are  1^  in.  in  diameter. 


h  h 


Fig.  189— Represents  a  horizontal  cross-section  of  the  T  irons  and  side  plates. 
Z),  cross-section  near  centre  of  bridge. 

cross-section  near  the  piers, 
fir,  plates  of  the  sides. 
A,  exterior  T  irons. 
<,  interior  T  irons. 


TUBULAR  BRIDGES. 


353 


.  The  sides  of  the  tube  (Fig.  188)  between  the  cellular  top 
and  bottom  are  formed  of  sheets  2  ft.  wide  ;  the  lengths  of 
whicli  are  so  arranged  that  there  are  alternately  three  and  four 
plates  in  each  panel,  the  sheets  of  each  panel  abutting  end 
to  end,  and  forming  a  continuous  vertical  joint  between  the 
adjacent  panels.  These  vertical  joints  are  secured  by  strips 
of  iron,  h  and  ^,  of  the  T  cross-section,  placed  over  each  side 
of  the  joint,  and  clamping  the  sheets  of  the  adjacent  parcels 
between  them.  The  T  irons  within  and  without  are  firmly 
riveted  together  with  1-inch  rivets,  placed  at  3  in.  between 
their  centres.  Over  the  joints,  between  the  ends  of  the  sheets 
in  each  panel,  pieces  *of  sheet  iron  are  placed  on  each  side, 
and  connected  by  rivets.  The  sheets  of  the  panels  at  the 
centre  of  the  tube  are  j^ths  of  an  inch  thick ;  they  increase 
to  xfths  to  within  about  10  ft.  of  the  piers,  where  their  thick- 
ness is  again  increased :  and  the  T  irons  are  here  also  increased 
in  thickness,  being  composed  of  a  strip  of  thick  sheet  iron, 
clamped  between  strips  of  angle  iron  which  extend  from  the 
top  to  the  bottom  of  the  joints.  The  object  of  this  increase 
of  thickness,  in  the  panels  and  T  irons  tit  the  piers,  is  to  give 
sufficient  rigidity  and  strength  to  resist  the  crushing  strain  at 
these  points. 

The  T  irons  on  the  interior  are  bent  at  toj)  and  bottom,  and 
extended  as  far  as  the  third  cell  from  the  sides  at  top,  and  to 
the  second  at  bottom.  The  projecting  rib  of  each  in  the 
angles  is  clamped  between  two  pieces,  of  sheet  iron,  to 
which  it  is  secured  by  rivets,  to  give  greater  stiffness  at  the 
angles  of  the  tube. 

The  arrangement  of  the  ordinary  T  irons  and  sheets  of  the 
panels  is  shown  in  cross-section  by  Fig.  189  ;  and  that  of  the 
like  parts  near  the  piers  by      same  Fig. 

For  the  purpose  of  giving  greater  stiffness  to  the  bottom, 
and  to  secure  fastenings  for  the  wooden  cross  sleepers  that 
support  the  longitudinal  *beams  on  which  the  rails  lie,  cross 
plates  of  sheet  iron,  half  an  inch  thick,  and  10  in.  in  depth, 
are  laid  on  the  bottom  of  the  tube,  from  side  to  side,  at  every 
fourth  rib  of  the  T  iron,  or  6  ft.  apart.  These  cross  plates  are 
secured  to  the  bottom  by  angle  iron,  and  are  riveted  also  to 
the  T  iron. 

The  tube  is  firmly  fixed  to  the  central  pier,  but  at  the  inter- 
mediate piers  and  the  abutments  it  rests  upon  saddles  sup- 
ported on  rollers  and  balls,  to  allow  of  the  play  from  con- 
traction and  expansion  by  changes  of  temperature. 

The  following  tabular  statements  give  the  details  of  the 
dimensions,  weights,  etc.,  of  the  Britannia  Bridge. 
23 


354 


CIVIL  ENGINEEEING. 


Total  length  of  each  tube  

"  of  tubes  for  each  line ....  

Greatest  span  of  bay  

Height  of  tubes  at  the  middle  

"  "       intermediate  piers  

"  "  ends  

Extreme  width  of  tubes  

Number  of  rivets  in  one  tube  

Computed  weight  of  tube  274  ft.  long  

"      3  tubes  274  ft.  long  

"  "1  tube  472  ft.  long  

"  "      3  tubes  472  ft.  long  

"  "      1  tube  over  pier  32  ft.  long 

Frames  and  beams  

Total  weight  


Feet. 


1524 
3U48 
4()0 
30 
27 
2S 

882,000 


450 
1350 
905 
2895 
64 
64 


109 

327 
188 
564 
26 


1240 


T  i  Bivet 
iron.  iron. 


tons.  tens. 


70 
210 
139 
417 
10 
10 


60 
180 
108 
324 
7 
7 


686 


Cast- 
iron. 


2000 


2000 


Total 


646.  Formula  for  reducing  the  Breaking-  Weight  of 

Wrought  Iron  Tubes. 

Representing  by  A,  the  total  area  in  inches  of  the  cross- 
section  of  the  metaL 
"  "    d,  the  total  depth  in  inches  of  the  tube. 

"  "    I,  the  length  in  inches  between  the  points 

of  support. 

"  "    C,  a  constant  to  be  determined  by  ex- 

periment. 
"  W,  the  breaking  weight  in  tons. 

Then  the  relations  between  these  elements,  in  tubes  of 
cylindrical,  elliptical  and  rectangular  cross-section,  will  be 
expressed  by 

The  mean  yalue  for  C  for  cylindrical  tubes,  deduced  from 
several  experiments,  was  fomid  to  be  13.03;  that  for  ellipti- 
cal tubes,  16.3 ;  and  that  for  rectangular  tubes,  21.5. 

647.  Victoria  Bridge.  This  bridge  is  located  near  Mon- 
treal. It  is  a  tubular  bridge,  a  cross-section  of  which  is  shown 
in  Fig.  190.  It  is  the  largest  bridge  of  its  kind  in  existence. 
It  consists  of  twenty-four  openings  of  242  feet  each,  and  a 
central  span  of  330  feet,  and  the  total  length  of  the  tube,  in- 
cluding the  width  of  the  abutments,  is  6,538.  The  em- 
bankment forming  the  approach  at  the  Alontreal  end  is  1,200 
feet  long,  and  at  the  south  end  it  is  800  feet,  making  a  total 
length,  including  the  approaches,  of  nearly  8,000  feet. 

The  centre  span  is  level,  but  each  side  of  the  centre  the 
bridge  falls  on  a  grade  of  40  feet  per  mile. 


TUBULAR  BRIDGES. 


355 


Pig.  190— Victoria  Bridge. 

Web  plates  and  top  plates  at  centre  of  tube. 

A,  tie  bar  6"  x  0".75 

B,  web  plate. 

C  C,  cover  plates. 

D,  top  plates. 

E,  bottom  plates. 

F,  heavy  wooden  beams  on  which  rail  H  rests, 
G-,  cross  timber  to  connect  beams  F. 


Fig.  190  A— is  an  enlarged  view  of  a 
part  of  one  of  the  upper  cells.  The 
letters  apply  to  the  same  parts  as  in 
the  preceding  Figure. 

A  is  the  top  plate. 

D  shows  two  continuous  plates,  and 
C  C,  two  joint  plates. 


Each  tube  covers  two  openings,  being  fixed  in  the  centre, 
and  free  to  expand  or  contract  on  the  adjacent  piers.  They 
are  16  feet  wide  and  19  feet  deep  at  their  ends,  and  gradually 


356 


CIVIL  ENGINEERING. 


Fig.  190  C — Section  of  the  bottom  plates  E  of  Fig.  190.  There  are  three  continuous  plates 
and  four  joint  plates. 


increase  in  depth  to  the  middle,  where  they  are  16  feet  wide 
by  21  feet  8  inches  deep.    The  total  length  of  each  of  these 


double  tubes  is, 

On  the  centre  pier   16  feet. 

Two  openings  in  the  clear  484  " 

Kesting  on  the  east  pier.   8  " 

Resting  on  the  west  pier   8  " 


Total   516  feet. 


The  weight  of  each  tube  of  516  feet  is  about  644  tons.  At 
each  end  are  seven  expansion  rollers,  each  6  inches  in  diame- 
ter, upon  which  the  tubes  rest.  The  rollers  which  are  turned 
rest  on  planed  cast-iron  bed  plates. 

The  centre  ;pier  is  24  feet  wide,  the  remaining  ones  each 
16  feet  wide  at  the  top. 

The  work  of  laying  the  foundation  was  begun  in  1854,  and 
the  centre  tube  was  put  in  place  in  March,  1859. 

The  scaffolding  for  the  centre  tube  rested  on  the  ice  in  the 
river,  wliich  began  to  move  the  day  after  the  tube  was  put 
in  place.  From  a  record  wliich  had  been  kept  of  the  break- 
ing up  of  the  ice,  it  was  presumed  that  it  would  remain  sound 
several  days  longer  than  it  did. 

The  foundations  were  made  on  the  solid  rock  by  means  of 
coffer-dams.  Two  kinds  were  used,  oue  a  floating  dam,  and 
the  other  a  permanent  crib-work  ;  and  each  possessed  certain 
advantages  over  the  other  which  was  peculiar  to  itself  and  to 
the  objects  which  w^ere  to  be  accomplished. 


YII. 


SUSPENSION  BRIDGES. 

648.  The  use  of  flexible  materials,  as  cordage  and  the  like, 
to  form  a  roadway  over  chasms  and  narrow  water-courses, 
dates  from  a  very  early  period;  and  structures  of  this  char- 
acter were  probably  among  the  first  rude  attempts  of  ingenu- 
ity, before  the  arts  of  tli^  carpenter  and  mason  were  suf- 
ficiently advanced  to  be  made  subservient  to  the  same  ends. 
The  idea  of  a  suspended  roadway,  in  its  simplest  form,  is  one 
that  would  naturally  present  itself  to  the  mind,  and  its  con- 
sequent construction  would  demand  only  obvious  means  and 
but  little  mechanical  contrivance  ;  but  the  step  from  this 
stage  to  the  one  in  which  such  structures  are  now  found, 
supposes  a  very  advanced  state  both  of  science  and  of  its 
application  to  the  industrial  arts,  and  we  accordingly  find 
that  bridge  architecture,  imder  every  other  guise,  was  brought 
to  a  high  degree  of  perfection  before  the  suspension  bridge, 
as  this  structure  is  now  understood,  was  attempted. 

With  the  exception  of  some  isolated  cases  which,  but  in  the 
material  employed,  differed  little  from  the  first  rude  struc- 
tures, no  recorded  attempt  had  been  made  to  reduce  to  syste- 
matic rules  the  means  of  suspending  a  roadway  now  in  use, 
until  about  the  year  1801,  when  a  patent  was  taken  out  in 
this  country  for  the  purpose,  by  Mr.  Finlay,  in  w^hich  the 
manner  of  hanging  the  chain  supports,  and  suspending  the 
roadway  from  it,  are  specifically  laid  down,  differing,  in  no 
very  material  point,  from  the  practice  of  the  present  day  in 
this  branch  of  bridge  architecture.  Since  then,  a  number 
of  structures  of  this  character  have  been  erected  both  in  the 
United  States  and  in  Europe,  and,  in  some  instances,  valleys 
and  water-courses  have  been  spanned  by  them  under  circum- 
stances which  would  have  baffled  the  engineer's  art  in  the  em- 
ployment of  any  other  means. 

A  suspension  bridge  consists  of  the  supports,  termed  j9^dr<s, 
from  which  the  suspension  chains  are  hung ;  of  the  anchoring 
masses,  termed  the  cibiitments^  to  which  the  ends  of  the  sus- 
pension chains  are  attached  ;  of  the  suspension  chains,  termed 
the  main  chains^  from  which  the  roadway  is  suspended ;  of 
the  vertical  rods,  or  chains,  termed  the  stisjp ending -chains^  etc., 
which  connect  the  roadway  with  the  main  chains;  and  of  the 
roadway. 

649.  Bays.    The  natural  water-way  may  be  divided  into 


358  CIVIL  ENGINEERING. 

any  number  of  equal-sized  bays,  depending  on  local  circum- 
stances, and  the  comparative  cost  of  high  or  low  piers,  and 
that  of  the  main  chains,  and  the  suspending-rods. 

A  bridge  with  a  single  bay  of  considerable  width  presents 
a  bolder  and  more  monumental  character,  and  its  stability, 
all  other  things  being  equal,  is  greater,  the  amplitude  from 
undulations  caused  by  a  movable  load  being  less  than  one  of 
several  bays. 

650.  A  chain  or  rope,  when  fastened  at  each  extremity  to 
fixed  points  of  support,  will,  from  the  action  of  gravity, 
assume  the  form  of  a  catenary  m  a  state  of  equilibrium, 
whether  the  two  extremities  be  on  the  same  or  different  levels. 
The  relative  height  of  the  fixed  supports  may  therefore  be 
made  to  conform  to  the  locality. 

651.  The  ratio  of  the  versed  sine  of  the  arc  to  its  chord,  or 
span,  will  also  depend,  for  the  most  part,  on  local  circum- 
stances and  the  object  of  the  suspended  structure.  The 
wider  the  span,  or  chord,  for  the  same  versed  sine,  the  greater 
will  be  the  tension  along  the  curve,  and  the  more  strength 
will  therefore  be  required  in  all  the  parts  of  the  cable.  The 
reverse  will  obtain  for  an  increase  of  versed  sine  for  the  same 
span  ;  but  there  will  be  an  increase  in  the  length  of  the  curve. 

652.  The  chains  may  either  be  attached  at  the  extremities 
of  the  curve  to  the  fixed  supports,  or  piers  ;  or  they  may  rest 
upon  them  (Figs.  191, 192),  being  fixed  into  anchoring  masses, 


Fig.  191— Represents  a  chain  arcli  abode,  resting  upon  two  piers//,  and  anchored  at  the 
points  a  and  e,  from  which  a  horizontal  beam  m  n  is  suspended  by  vertical  chains,  or  rods. 


ii 

fllliTiTtm^^ 

1 

m 

/ 

\\\\\v 

MM 

1 

Fig.  193 — ^Represents  the  manner  in  which  the  system  may  be  arranged  when  a  single  pier  is 
placed  between  the  extreme  points  of  the  bearing. 


or  abutments,  at  some  distance  beyond  the  piers.  Local 
circumstances  will  determine  which  of  the  two  methods  will 
be  the  more  suitable.  The  latter  is  generally  adopted,  partic- 
ularly if  the  piers  require  to  be  high,  since  the  strain  upon 
tliem  from  the  tension  might,  from  the  leverage,  cause  rup- 


SUSPENSION  BRIDGES. 


359 


ture  in  the  pier  near  the  bottom,  and  because,  moreover,  it 
remedies  in  some  degree  the  inconveniences  arising  from 
variations  of  tension  caused  either  by  a  movable  load  or 
changes  of  temperature.  Piers  of  wood,  or  of  cast  iron, 
movable  around  a  joint  at  their  base,  have  been  used  instead 
of  fixed  piers,  with  the  object  of  remedying  the  same  incon- 
veniences. 

653.  When  the  chains  pass  over  the  piers  and  are  anchored 
at  some  distance  beyond  them,  they  may  either  rest  upon 
saddle-pieces  of  cast  iron,  or  upon  pulleys  placed  on  the 
piers. 

654.  The  position  of  the  anchoring  points  will  depend  upon 
local  circumstances.  The  two  branches  of  the  chain  may 
either  make  equal  angles  with  the  axis  of  the  pier,  thus  assum- 
ing the  same^:  curvature  on  each  side  of  it,  or  else  the  extrem- 
ity of  the  chain  may  be  anchored  at  a  point  nearer  to  the  base 
of  the  pier.  In  the  former  case  the  resultant  of  the  tensions 
and  weights  will  be  vertical  and  in  the  direction  of  the  axis 
of  the  pier,  in  the  latter  it  will  be  oblique  to  the  axis,  and 
should  pass  so  far  within  the  base  that  the  material  will  be 
secure  from  crushing.  When  the  cable  is  secured  to  a  sad- 
dle, and  the  saddle  is  free  to  move  horizontally  on  the  top  of 
the  pier,  the  resultant  forces  would  still  be  vertical  if  there 
were  no  frictional  resistance  to  the  movement  of  the  saddle. 
In  all  cases,  whether  the  inclinations  of  the  cable  on  the  oppo- 
site sides  of  the  pier  are  equal  or  not,  the  frictional  resistance 
under  the  saddle  when  it  is  moving  w^ill  cause  a  horizontal 
force  tending  to  overturn  the  pier. 

655.  The  anchoring  points  are  usually  masses  of  masonry 
of  a  suitable  form  to  resist  the  strain  to  which  they  are  sub- 
jected. They  may  be  placed  either  above  or  below  the  sur- 
face of  the  ground,  as  the  locality  may  demand.  The  kind 
of  resistance  offered  by  them  to  the  tension  on  the  chain  will 
depend  upon  the  position  of  the  chain.  If  the  two  branches 
of  the  chain  make  equal  angles  with  the  axis  of  the  pier,  the 
resistance  offered  by  the  abutments  will  mainly  depend  upon 
the  strength  of  the  material  of  which  they  are  formed.  If 
the  branches  of  the  chain  make  unequal  angles  with  the  axis 
of  the  pier,  the  branch  fixed  to  the  anchoring  mass  is  usually 
deflected  in  a  vertical  direction,  and  so  secured  that  the  weight 
of  the  abutment  may  act  in  resisting  tlie  tension  on  the  chain. 
In  this  plan  fixed  pulleys  placed  on  very  firm  supports  will 
be  required  at  the  point  of  deflection  of  the  chain  to  resist  the 
pressure  arising  from  the  tension  at  these  points. 

Whenever  it  is  practicable  the  abutment  and  pier  should  be 


360 


CIVIL  ENGINEERING. 


suitably  connected  to  increase  the  resistance  offered  by  the 
former. 

The  connection  between  the  chains  and  abutments  should 
be  so  arranged  that  the  parts  can  be  readily  examined.  The 
chains  at  these  points  are  sometimes  imbedded  in  a  paste  of 
fat  lime  to  preserve  them  from  oxidation. 

656.  The  chains  may  be  placed  either  above  or  below  the 
structure  to  be  supported.  The  former  gives  a  system  of 
more  stability  than  the  latter,  owing  to  the  position  of  the 
centre  of  gravity,  but  it  usually  requires  high  piers,  and  the 
chain  cannot  generally  be  so  well  arranged  as  in  the  latter  to 
subserve  the  required  purposes.  The  curves  may  consist  of 
one  or  more  chains.  Several  are  usually  preferred  to  a  single 
one,  as  for  the  same  am.ount  of  metal  they  offer  more  resist- 
ance, can  be  more  accurately  manufactured,  are  less  liable  to 
accidents,  and  can  be  more  easily  put  up  and  replaced  than  a 
single  chain.  The  chains  of  the  curve  may  be  placed  either 
side  by  side,  or  above  each  other,  according  to  circumstances. 

657.  The  cables  may  be  formed  either  of  chains,  of  wire 
cables,  or  of  bands  of  hoop  iron.  Each  of  these  methods  has 
found  its  respective  advocates  among  engineers.  Those  who 
prefer  wire  cables  to  chains  urge  that  the  latter  are  more 
liable  to  accidents  than  the  former,  that  their  strength  is  less 
uniform  and  less  in  proportion  to  their  weight  than  that  of 
wire  cables,  that  iron  bars  are  more  liable  to  contain  con- 
cealed defects  than  wire,  that  the  proofs  to  which  chains  are 
subjected  may  increase  without,  in  all  cases,  exposing  these 
defects,  and  that  the  construction  and  putting  up  of  cliains  is 
more  expensive  and  difficult  than  for  wire  cables.  The  op- 
ponents of  wire  cables  state  that  they  are  open  to  the  same 
objections  as  those  urged  against  chains,  that  the}"  offer  a 
greater  amount  of  surface  to  oxidation  than  the  same  volume 
of  bar  iron  would,  and  that  no  precaution  can  prevent  the 
moisture  from  penetrating  into  a  wire  cable  and  causing  rapid 
oxidation. 

That  in  this,  as  in  all  like  discussions,  an  exaggerated  de- 
gree of  importance  should  have  been  attached  to  the  objec- 
tions urged  on  each  side  was  but  natural.  Experience,  how- 
ever, derived  from  existing  works,  has  shown  that  each 
method  may  be  applied  with  safety  to  structures  of  the 
boldest  character,  and  that  wherever  failures  have  been  met 
with  in  either  method,  they  were  attributable  to  those  faults 
of  workmanship,  or  to  defects  in  the  material  used,  which 
can  hardly  be  anticipated  and  avoided  in  any  novel  applica- 
tion of  a  like  character.    Time  alone  can  definitively  decide 


SUSPENSION  BEIDGES. 


361 


upon  the  comparative  merits  of  the  two  methods,  and  how 
far  either  of  them  may  be  used  with  advantage  in  the  place 
of  structures  of  more  rigid  materials. 

658.  The  chains  of  the  curves  may  be  formed  of  either 
round,  square,  or  flat  bars.  Chains  of  flat  bars  have  been 
most  generally  used.  These  are  formed  in  long  links  which 
are  conuected  by  short  plates  and  bolts.  Each  link  consists 
of  several  bars  of  the  same  length,  each  of  which  is  perforated 
with  a  hole  at  each  end  to  receive  the  connecting  bolts.  The 
bars  of  each  link  are  placed  side  by  side,  and  the  links  are 
connected  by  the  plates  which  form  a  short  link,  and  the  bolts. 

The  links  of  the  portions  of  the  chain  which  rest  upon  the 
piers  may  either  be  bent,  or  else  be  made  shorter  than  the 
others  to  accommodate  the  chain  to  the  curved  form  of  the 
surface  on  which  it  rests. 

659.  T]ie  vertical  suspension  bars  may  be  either  of  round 
or  square  bars.  They  are  usually  made  with  one  or  more 
articulations,  to  admit  of  their  yielding  Avith  less  strain  to  the 
bar  to  any  motion  of  vibration  or  of  oscillation.  They  may 
be  suspended  from  the  connecting  bolts  of  the  links,  but  the 
preferable  method  is  to  attach  them  to  a  suitable  saddle-piece 
which  is  fitted  to  the  top  of  the  chain  and  thus  distributes  the 
strain  upon  the  bar  more  uniformly  over  the  bolts  and  links. 
The  lower  end  of  the  bar  is  suitably  arranged  to  connect  it 
with  the  part  suspended  from  it. 

660.  The  wire  cables  are  composed  of  wires  laid  side  by 
side,  which  are  brought  to  a  cylindrical  shape  and  confined 
by  a  spiral  wrapping  of  wire.  To  form  the  cable  several 
equal-sized  ropes,  or  yarns,  are  first  made.  This  may  be 
done  by  cutting  all  the  wires  of  the  length  required  for  the 
yarn,  or  by  uniting  end  to  end  the  requisite  number  of  wires 
for  the  yarn,  and  then  winding  them  around  two  pieces  of 
wrought  or  of  cast  iron,  of  a  horse-shoe  shape,  with  a  suitable 
gorge  to  receive  the  wires,  which  are  placed  as  far  asunder 
as  the  required  length  of  the  yarn.  The  yarn  is  firmly 
attached  at  its  two  ends  to  the  iron  pieces,  or  criq^pers,  and 
the  wires  are  temporarily  confined  at  intermediate  points  by 
a  spiral  lashing  of  wire.  Whichever  of  the  two  methods  be 
adopted,  great  care  must  be  taken  to  give  to  every  wire  of  the 
yarn  the  same  degree  of  tension  by  a  suitable  mechanism. 
The  cable  is  completed  after  the  yarns  are  placed  upon  the 
piers  and  secured  to  the  anchoring  ropes  or  chains ;  for  this 
purpose  the  temporary  lashings  of  the  yarns  are  undone,  and 
all  the  yarns  are  united  and  brought  to  a  cylindrical  shape 
and  secured  throughout  the  extent  of  the  cable,  to  within  a 

I 


362 


CIVIL  ENGINEEEING. 


short  distance  of  each  pier,  hy  a  continuous  spiral  lashing  of 
wire. 

The  part  of  the  cable  which  rests  upon  the  pier  is  not 
bound  with  wire,  but  is  spread  over  the  saddle-piece  with  a 
uniform  thickness. 

661.  The  suspension  ropes  are  formed  in  the  same  way  as 
the  cables ;  they  are  usually  arranged  with  a  loop  at  each  end, 
fonned  around  an  iron  crupper,  to  connect  them  with  the 
cables,  to  wJiich  they  are  attached,  and  to  the  parts  of  the 
structure  suspended  from  them  by  suitable  saddle-pieces. 

662.  To  secure  the  cables  from  oxidation  the  iron  wires  are 
coated  with  varnish  before  they  are  made  into  yarns,  and 
after  the  cables  are  completed  they  are  either  coated  with  the 
usual  paints  for  securing  iron  from  the  effects  of  moisture,  or 
else  covered  with  some  impermeable  material. 

663.  Piers.  These  are  commonly  masses  of  masonry  in  the 
shape  of  pillars,  or  columns,  that  rest  on  a  common  foundation, 
and  are  usually  connected  at  the  top.  The  form  given  to  the 
pier,  when  of  stone,  will  depend  in  some  respects  on  the 
locality.  Generally  it  is  that  of  the  architectural  monument 
known  as  the  Tri%i7n])lial  Arch;  an  arched  opening  being 
formed  in  the  centre  of  the  mass  for  the  roadway,  and  some- 
times two  others  of  smaller  dimensions,  on  each  side  of  the 
main  one,  for  approaches  to  the  footpatlis  of  the  bridge. 

Piers  of  a  columnar,  or  of  an  obelisk  form,  have  in  some 
instances  been  ti-ied.  They  have  generally  been  found  to  be 
wanting  in  stiffness,  being  subject  to  vibrations  from  the 
action  of  the  chains  upon  them,  Avhich  in  turn,  from  the  re- 
ciprocal action  upon  the  chains,  tends  very  much  to  increase 
the  amplitude  of  the  vibrations  of  the  latter.  These  effects 
have  been  observed  to  be  the  more  sensible  as  the  columnar 
piers  are  the  higher  and  more  slender. 

Cast-iron  piers,  in  the  form  of  columns  connected  at  top  by 
an  entablature,  have  been  tried  with  success,  as  also  have  been 
columnar  piers  of  the  same  material  so  arranged,  with  a  joint 
at  their  base,  that  they  can  receive  a  pendulous  motion  at  top 
to  accommodate  any  increase  of  tension  upon  either  branch 
of  the  chain  resting  on  them. 

The  dimensions  of  piers  will  depend  upon  their  height  and 
the  strain  upon  them.  When  built  of  stone,  the  masonry 
should  be  very  carefully  constructed  of  large  blocks  well 
bonded,  and  tied  by  metal  cramps.  The  height  of  the  piers 
will  depend  mostly  on  the  locality.  When  of  the  usual  forms, 
they  should  at  least  be  high  enough  to  admit  the  passage  of 
vehicles  under  the  arched  way  of  the  road. 


SUSPENSION  BRIDGES. 


363 


664.  Abutments.  The  forms  and  dimensions  of  the  abut- 
ments will  depend  upon  the  manner  in  which  they  may  be 
connected  with  the  chains.  When  the  locality  will  admit  of  the 
chains  being  anchored  without  deflecting  them  vertically,  the 
al)utments  may  be  formed  of  any  heavy  mass  of  rough 
masonry,  which,  from  its  weight,  and  the  manner  in  which  it 
is  imbedded,  have  sufficient  strength  to  resist  the  tension  in 
the  direction  of  the  chain.  If  it  is  found  necessary  to  deflect 
the  chains  vertically  to  secure  a  good  anchoring  point,  it  will 
also  generally  be  necessary  to  build  a  mass  of  masonry  of  an 
arched  form  at  the  point  where  the  deflection  takes  place, 
which,  to  present  sufficient  strength  to  resist  the  pressure 
caused  by  the  resultant  of  the  tension  on  the  two  branches  of 
the  chain,  should  be  made  of  heavy  blocks  of  cut  stone  w^ell 
bonded.  If  the  abutments  are  not  too  far  from  the  founda- 
tions of  the  piers,  it  will  be  well  to  connect  the  two,  in  order 
to  give  additional  resistance  to  the  anchoring  points. 

665.  Main  Chains,  etc.  The  suspending  curves,  or  arches,, 
may  be  made  of  chains  formed  of  fiat  or  round  iron,  or  may 
consist  of  wire  cables  constructed  in  the  usual  manner. 

The  main  chains  of  the  earlier  suspension  bridges  were 
formed  of  long  links  of  round  iron  made  in  the  usual  way  ; 
but,  independently  of  the  greater  expense  of  these  chains, 
they  were  found  to  be  liable  to  defects  of  welding,  and  the' 
links,  when  long,  were  apt  to  become  misshapen  imder  a  great 
strain,  and  required  to  be  stayed  to  preserve  their  lorm^. 
Chains  formed  of  long  links  of  flat  bars,  usually  connected  bj 
shorter  ones,  as  coupling  links,  have  on  these  accounts  super- 
seded those  of  the  ordinary  oval-shaped  links. 

The  breadth  of  the  chains  has  generally  been  made  uniform,, 
but  in  some  bridges  erected  in  England  by  Mr.  Dredge,  the 
chains  are  made  to  increase  uniformly  in  breadth,  by  increas- 
ing the  number  of  bars  in  a  link,  from  the  centre  to  the  points 
of  suspension.  In  addition  to  this  change  in  the  form  of  the 
main  chains,  Mr.  Dredge  places  the  suspending  chains  in  a 
vertical  plane  parallel  to  the  axis  of  the  bridge,  but  obliquely 
to  the  horizon,  inclining  each  w^ay  from  the  points  of  suspen- 
sion towards  the  centre  of  the  curve.  This  system  has  never 
come  into  general  use.  At  the  present  day  nearly  all  cables 
of  suspension  bridges  are  made  of  wire. 

Some  of  the  links  of  the  main  chains  should  be  arranged 
with  adjusting  screws,  or  with  keys,  to  bring  the  chains  to  the 
proper  degree  of  curvature  when  set  up. 

The  chains  may  either  be  attached  to,  or  pass  over  a  mo- 
vable cast-iron  saddle,  seated  on  rollers  on  the  top  of  the  piers,. 


364: 


CIVIL  ENGINEERING. 


SO  that  it  will  allow  of  sufficient  horizontal  displacement  to 
permit  the  chains  to  accommodate  themselves  to  the  effects 
of  a  movable  load  on  the  roadway.  The  same  ends  may  be 
attained  by  attaching  the  chains  to  a  pendulum  bar  suspended 
from  the  top  of  the  pier. 

The  chains  are  firmly  connected  with  the  abutments,  by 
being  attached  to  anchoring  masses  of  cast  iron,  arranged  in 
a  suitable  manner  to  receive  and  secure  the  ends  of  the 
chains,  which  are  carefully  imbedded  in  the  masonry  of  the 
abutments.  These  points,  when  under  ground,  should  be  so 
placed  that  they  can  be  visited  and  examined  fi-om  time  to 
time. 

666.  Suspending-Chains.  The  suspending-rods,  or  chains, 
should  be  attached  to  such  points  of  the  main  chains  and  the 
roadway-bearers,  as  to  distribute  the  load  uniformly  over  the 
main  chains,  and  to  prevent  their  being  broken  or  twisted  off 
by  the  oscillations  of  the  bridge  from  winds,  or  movable 
loads.  They  should  be  connected  by  suitably -arranged  ar- 
ticulations, with  a  saddle  piece  bearing  upon  the  back  of  the 
main  chain,  and  at  bottom  with  the  stirrup  that  embraces  the 
roadway-bearers. 

The  suspending-chains  are  nsually  hung  vertically.  In 
some  recent  bridges  they  have  been  inclined  inw^ard  to  give 
more  stiffness  to  the  system. 

667.  Roadway.  Transversal  road  way -bearers  are  attached 
to  the  suspending-chains,  upon  which  a  flooring  of  timber  is 
laid  for  the  roadway.  The  roadway-bearers,  in  some  in- 
stances, have  been  made  of  wrought  iron,  but  timber  is  now 
generally  preferred  for  these  pieces.  Diagonal  ties  of 
wrought  iron  are  placed  horizontally  betw^een  the  roadway- 
bearers  to  brace  the  fram^-work. 

The  parapet  may  be  formed  in  the  usual  style  either  of 
wrought  iron,  or  of  timber,  or  of  a  combination  of  cast  iron 
and  timber.  Timber  alone,  or  in  combination  with  cast  iron, 
is  now  preferred  for  the  parapets  ;  as  observation  has  shown 
that  the  stiffness  given  to  the  roadway  by  a  strongly-trussed 
timber  parapet  limits  the  amplitude  of  the  undulations  caused 
by  violent  w^nds,  and  secures  the  structure  from  danger. 

In  some  of  the  more  recent  suspension  bridges,  a  trussed 
frame,  similar  to  the  parapet,  has  been  continued  below  the 
level  of  the  roadway,  for  the  purpose  of  giving  greater  se- 
curity to  the  structure  against  the  action  of  high  winds. 

When  the  roadway  is  above  the  chains,  any  requisite  num- 
ber of  single  chains  may  be  placed  for  its  support.  Frames 
formed  of  vertical  beams  of  timber,  or  of  columns  of  cast 


SUSPENSION  BRIDGES. 


365 


ii'Oii  united  by  diagonal  braces,  rest  upon  the  chains,  and  sup- 
port the  roadway-bearers  placed  either  transversely  or 
longitudinally.  . 

6^8.  Vibrations.  The  undulatory  or  vibratory  motions  of 
suspension  bridges,  caused  by  the  action  of  high  winds,  or 
movable  loads,  should  be  reduced  to  the  smallest  practicable 
amount,  by  a  suitable  arrangement  of  bracing  for  the  road- 
way-timbers and  parapet,  and  by  chain-stays  attached  to  the 
roadway  and  to  the  basements  of  the  piers,  or  to  fixed  points 
on  the  banks  whenever  they  can  be  obtained. 

Calculation  and  experience  show  that  the  vibrations  caused 
by  a  movable  load  decrease  in  amplitude  as  the  span  in- 
creases, and,  for  the  same  span,  as  the  versed  sine  decreases. 
The  heavier  the  roadway,  also,  all  other  things  being  the  same, 
the  smaller  will  be  the  amplitude  of  the  vibrations  caused  by 
a  movable  load,  and  the  less  will  be  their  effect  in  changing 
the  form  of  the  bridge. 

The  vibrations  caused  by  a  movable  load  seldom  affect  the 
bridge  in  a  hurtful  degree,  owing  to  the  elasticity  of  the 
system,  unless  they  recur  periodically,  as  in  the  passage  of  a 
body  of  soldiers  with  a  cadenced  march.  Serious  accidents 
have  been  occasioned  in  this  way ;  also  by  the  passage  of  cat- 
tle, and  by  the  sudden  rush  of  a  crowd  from  one  side  of  the 
bridge  to  the  other.  Injuries  of  this  character  can  only  be 
guarded  against  by  a  proper  system  of  police  regulations. 

Chain-stays  may  either  be  attached  to  some  point  of  the 
roadway  and  to  fixed  points  beneath  it,  or  else  they  may  be  in 
the  form  of  a  reversed  curve  below  the  roadway.  The  former 
is  the  more  efticacious,  but  it  causes  the  bridge  to  bend  in  a 
disagreeable  manner  at  the  point  where  the  stay  is  attached, 
when  the  action  of  a  movable  load  causes  the  mxain  chains  to 
rise.  The  more  oblique  the  stays,  the  longer,  more  expensive, 
and  less  effective  they  become.  Stays  in  the  form  of  a  re- 
versed curve  preserve  better  the  shape  of  the  roadway  under 
the  action  of  a  movable  load,  but  they  are  less  effective  in 
preventing  vibrations  than  the  simple  stay.  Neither  of  these 
methods  is  very  serviceable,  except  in  narrow  spans.  In  wide 
spans,  variations  of  temperature  cause  considerable  changes 
in  the  length  of  the  stays,  which  makes  them  act  unequally 
upon  the  roadway ;  this  is  particularly  the  case  with  the  re- 
veiled  curve.  Both  kinds  should  be  arranged  with  adjusting 
screws,  to  accommodate  their  length  to  the  more  extreme 
variations  of  tempel'ature. 

Engineers  at  present  generally  agree  that  the  most  eflfica- 
cious  means  of  limiting  the  amplitude,  and  the  consequent 


366 


CIVIL  ENGINEERING. 


injurious  effects  of  undulations,  consists  in  a  strong  combina- 
tion of  the  roadway-timbers  and  flooring,  stiffened  l)y  a  trussed 
parapet  of  timber  above  the  roadway,  and  in  some  cases  in 
extending  the  framework  of  the  parapet  below  it.  These 
combinations  present,  in  appearance  and  reality,  two  or  more 
023en-built  beams,  as  circumstances  may  demand,  placed  paral- 
lel to  each  other,  and  strongly  connected  and  braced  by  the 
framework  of  the  roadway,  which  ai'e  supported  at  inter- 
mediate points  by  the  suspending- rods  or  chains.  The 
method  of  placing  the  roadway-framing  at  the  central  line  of 
the  open-built  beams,  presents  the  advantage  of  introducing 
vertical  diagonal  braces,  or  ties,  between  the  beams  beneath 
the  roadway-frame.  The  main  objections  to  these  combina- 
tions is  the  increased  tension  thrown  upon  the  chains  from 
the  greater  weight  of  the  framework.  This  increase  of  ten- 
sion, however,  provided  it  be  kept  within  proper  limits,  so  far 
from  being  injurious,  adds  to  the  stability  and  security  of  the 
bridge,  both  from  the  effects  of  undulations  and  of  vibra- 
tions from  shocks. 

As  a  farther  security  to  the  stability  of  the  structure,  the 
framevrork  of  the  roadway  should  be  firmly  attached  at  the 
two  extremities  to  the  basements  of  the  piers. 

669.  Preservative  Means.  To  preserve  the  chains  from 
oxidation  on  the  surface,  and  from  rain  or  dews  whicli  may 
lodge  in  the  articulations,  they  should  receive  several  coats  of 
minium,  or  of  some  other  pre2:)aration  impervious  to  water, 
and  this  should  be  renewed  from  time  to  time,  and  the  forms 
of  all  tlie  parts  should  be  the  most  suitable  to  allow  the  free 
esca]:)e  of  moisture. 

Wires  for  cables  can  be  preserved  from  oxidation,  until  they 
are  made  into  ropes,  by  keeping  them  immersed  in  some  alka- 
line solution.  Before  making  them  into  ropes,  tliey  should 
be  dipped  several  times  in  boiling  linseed  oil,  prepared  by 
previously  boiling  it  with  a  small  portion  of  litharge  and 
lampblack.  The  cables  should  receive  a  thick  coating  of  the 
same  preparation  before  they  are  put  up,  and  finally  be 
painted  with  white-lead  paint,  both  as  a  preservative  means, 
and  to  show  any  incipient  oxidation,  as  the  rust  will  be  de- 
tected by  its  discoloring  the  paint. 

670.  Proofs  of  Suspension  Bridges.  From  the  many 
grave  accidents,  accomj^anied  by  serious  loss  of  life,  which 
have  taken  place  in  suspension  bridges,  it  is  highly  desirable 
that  some  trial-proof  should  be  made  before  opening  such 
bridges  to  the  public,  and  that,  moreover,  strict  police  regu- 
lations should  be  adopted  and  enforced,  with  respect  to  them, 


SUSPENSION  BEtDGES. 


367 


to  guard  against  the  recurrence  of  such  disasters  as  have  seve- 
ral times  taken  place  in  England,  from  the  assemblage  of  a 
crowd  upon  the  bridge.  In  France,  and  on  the  continent 
generally,  where  one  of  the  important  duties  of  the  public 
police  is  to  watch  over  the  safety  of  life,  under  such  circum- 
stances, regulations  of  this  character  are  rigidly  enforced. 
The  trial-proof  enacted  in  France  for  suspension  bridges,  be- 
fore they  are  thrown  open  for  travel,  is  about  40  lbs.  to  each 
superficial  foot  of  roadway  in  addition  to  the  permanent 
weight  of  the  bridge.  This  proof  is  at  first  reduced  to  one- 
half,  in  order  not  to  injure  the  masonry  of  the  points  of  sup- 
port during  the  green  condition  of  the  mortar.  It  is  made 
by  distributing  over  the  road  surface  any  convenient  weighty 
material,  as  bricks,  pigs  of  iron,  bags  of  earth,  etc.  Besides 
this  after-trial,  each  element  of  the  main  chains  should  be 
subjected  to  a  special  pro'of  to  prevent  the  introduction  of  un- 
sound parts  into  the  system.  This  precaution  will  not  be 
necessary  for  the  wire  of  a  cable,  as  the  process  of  drawing 
alone  is  a  good  test.  Some  of  the  coils  tested  will  be  a  guar- 
antee for  the  whole. 

From  experiments  made  at  Geneva,  by  Colonel  Dufour,  one 
of  the  earliest  and  most  successful  constructors  of  suspension 
bridges  on  the  Continent,  it  appears  that  WTOught  bar  iron 
can  sustain,  without  danger  of  rupture,  a  shock  arising  from 
a  weight  of  44  lbs.,  raised  to  a  height  of  3.28  feet  on  each, 
.0015dths  of  an  inch  of  cross-section,  when  the  bar  is  strained 
by  a  weight  equal  to  one-third  of  its  breaking  weight ;  and  he 
concludes  that  no  apprehension  need  be  entertained  of  injury 
to  a  bridge  from  shocks  caused  by  the  ordinary  transit  upon 
it,  which  has  been  subjected  to  the  usual  trial  of  a  dead  weight ; 
and  that  the  safety,  in  this  respect,  is  the  greater  as  the  bridge 
is  longer,  since  the  elasticity  of  the  system  is  the  best  pre- 
servative from  accidents  due  to  such  causes.  Mr.  Whoeler, 
an  engineer  in  Germany,  concluded,  after  a  long  series  of 
carefully  conducted  experiments,  that  good  wrought  iron 
Vould  sustain  any  number  of  continuous  shocks,  provided 
that  it  was  in  no  case  strained  more  than  10,000  pounds  per 
square  inch  of  section. 

671.  Durability.  Time  is  the  true  test  of  the  durability 
of  the  structures  under  consideration.  So  far  as  experience 
goes  there  seems  to  be  no  reason  to  assign  less  durability  to 
suspension  than  to  cast-iron  or  even  stone  bridges,  if  their  re- 
pairs and  the  proper  means  of  preserving  them  from  decay 
are  attended  to.  Doubts  have  been  expressed  as  to  the  dura- 
bility of  wire  cables,  but  these  seem  to  have  been  set  at  rest 


368 


CIVIL  ENGINEERING. 


by  the  trials  and  examinations  to  which  a  bridge  of  this  kind, 
erected  by  Colonel  Dufour,  at  Geneva,  was  subjected  by  him 
after  twenty  years'  service.  It  was  found  that  the  undulations 
were  greater  than  when  the  bridge  was  first  erected,  owing  to 
the  shrinking  of  the  roadway-frame ;  but  the  main  cables, 
and  suspending-ropes,  even  at  the  loops  in  contact  with  the 
timber,  proved  to  be  as  sound  as  when  first  put  up,  and  free 
from  oxidation  ;  and  the  whole  bridge  stood  another  very 
severe  proof  without  injury. 

The  following  succinct  descriptions  of  the  princij^al  ele- 
ments of  some  of  the  most  celebrated  suspension  bridges  of 
chains,  and  wire  cables,  of  remarkable  span,  are  taken  from 
various  published  accounts. 

672.  Bridge  over  the  Tweed  near  Berwick.  This  is  the 
first  large  suspension  bridge  erected  in  Great  Britain.  It 
was  constructed  upon  the  plans  of  Capt.  Brown^  who  took 
out  a  patent  for  the  principles  of  its  construction. 

Span   449  feet. 

Yersed  sine   30  " 

Number  of  main-chains  12,  six  being  placed  on  each  side  of 
the  roadway,  in  three  ranges  of  two  chains  each,  above 
each  other. 

The  chains  are  composed  of  long  links  of  round  iron,  2 
inches  in  diameter,  and  are  15  feet  long.  They  are  connected 
by  coupling-links  of  round  iron,  1^  inch  diameter,  and 
about  7  inches  long,  by  means  of  coupling  bolts. 

The  roadway  is  borne  by  suspending-rods  of  round  iron, 
which  are  attached  alternately  to  the  three  j-anges  of  chains. 
The  roadway-bearers  are  of  timber,  and  are  laid  upon  longi- 
tudinal bars  of  wrought  iron,  which  are  attached  to  the  sus- 
pension-rods. 

673.  Menai  Bridge,  erected  after  the  designs  of  Mr.  Tel- 
ford.    Opened  in  1826. 

Span   579.8  feet. 

Yersed  sine   43  " 

Number  of  main-chains  16,  arranged  in  sets  of  4  each,  ver- 
tically above  each  other. 
Number  of  bars  in  each  link,  5. 
Length  of  links,  10  feet. 

Breadth  of  each  bar,  3J  inches  ;  depth,  1  inch. 
Coupling-links,  16  inches  long,  8  inches  broad,  and  1  inch 
deep. 

Coupling-bolts,  3  inches  in  diameter. 

Total  area  of  cross-section  of  the  main-chain,  260  square 
inches. 


SrSPENSION  BRIDGES. 


369 


The  main-chains  are  fastened  to  their  abutments  by  an- 
choring-bolts  9  feet  long  and  6  inches  in  diameter,  which  are 
secured  in  cast-iron  grooves.  The  abutments,  which  are  un- 
derground, and  reached  by  suitable  tunnels,  are  the  solid  rock. 

Upon  the  tops  of  the  piers  are  cast-iron  saddles,  upon 
which  the  main-chains  rest.  The  base  of  the  saddle,  which 
is  fitted  with  grooves  to  receive  them,  rests  upon  iron  rollers 
placed  on  a  convex  cylindrical  bed  of  cast  iron,  shaped  like 
the  bottom  of  the  base  of  the  saddle,  to  admit  of  a  slight 
displacement  of  the  chains  from  movable  loads  or  changes 
of  temperature. 

The  roadway  is  divided  into  two  carriage-ways,  each  12 
feet  wide,  and  a  footpath  4  feet  wide  between  them.  The 
roadway-framing  consists  of  444  wrought-iron  roadway- 
bearers,  3 J-  inches  deep  and  ^  inch  thick,  which  are  sup- 
ported at  the  centre  points  of  each  of  the  carriage-ways  by 
an  inverted  truss,  consisting  of  two  bent  iron  ties  which  sup- 
port a  vertical  bar  placed  under  the  roadway-bars  at  the 
points  just  mentioned.  The  platform  of  the  roadway  is 
formed  of  two  thicknesses  of  plank.  The  first,  3  inches  thick^ 
is  laid  on  the  roadway-bearers  and  fastened  to  them.  This 
is  covered  by  a  coating  of  patent  felt  soaked  in  boiling  tar. 
The  second  is  two  inches  thick  and  spiked  to  the  first. 

The  roadway  is  suspended  by  articulated  rods  attached  to 
stirrups  on  the  roadway-bearers  and  to  the  coupling-bolts  o£ 
the  main-chains. 

The  piers  are  152  feet  high  above  the  high-water  level.. 
They  have  an  arched  opening  leading  to  the  roadway,  and 
the  masses  on  the  sides  of  the  arch  are  built  hollow,  with  a. 
cross-tie  partition  wall  between  the  exterior  main  walls. 

The  parapet  is  of  wrought-iron  vertical  and  parallel  bars 
connected  by  a  network. 

This  bridge  was  seriously  injured  by  a  violent  gale,  which 
gave  so  great  an  oscillation  to  the  main-chains  that  they  were 
dashed  against  each  other,  and  the  rivet-heads  of  the  bolts 
were  broken  off.  To  provide  against  similar  accidents,  a 
framework  of  cast-iron  tubes,  connected  by  diagonal  pieces, 
was  fastened  at  intervals  between  the  main-chains,  by  cross- 
ties  of  wrought-iron  rods,  which  passed  through  the  tubes, 
and  were  firmly  connected  with  the  exterior  chains.  Subse- 
quently to  this  addition,  a  number  of  strong  timber  roadway- 
bearers  were  fastened  at  intervals  to  those  of  iron,  as  the 
iron  roadway-bearers  were  found  to  have  been  bent,  and  in 
some  instances  broken,  by  the  undulatory  motion  of  the- 
bridge  in  heavy  gales. 
24 


370 


CIVIL  ENGINEERING. 


The  total  suspending  weight  of  this  bridge,  including  the 
main-chains,  roadway,  and  all  accessories,  is  stated  at  643 
tons  15^  cwt. 

^  674.  The  Fribourg  Bridge  of  wire  thrown  across  the 
I  valley  of  the  Sarine,  opposite  Fribourg,  was  erected  in  1832, 
I  by  M.  Chaley,  a  French  engineer. 

Span   870.32  feet. 

'  Yersed  sine   63.26  " 

There  are  4  main  cables,  2  on  each  side  of  the  road,  of 
the  same  elevation,  and  about  1^  inch  asunder.  Each  cable 
is  composed  of  1056  wires,  each  about  0.118  inch  in  diameter, 
which  are  firmly  connected  and  brought  to  cylindrical  shape 
by  a  spiral  wire  wrapping.  The  diameter  of  the  cable  varies 
from  5  to  6|-  inches.  The  cables  pass  over  3  fixed  pulleys  on 
the  top  of  the  piers,  upon  which  they  are  spread  out  without 
ligatures,  and  are  each  attached  to  two  other  cables  of  half 
their  diameter,  which  are  anchored  at  some  distance  from  the 
piers,  in  vertical  pits,  passing  over  a  fixed  pulley  where  they 
enter  the  mouth  of  the  pit. 

The  suspending-ropes  are  of  wire  a  size  smaller  than  that 
used  for  the  cables.  Their  diameter  is  nearly  one  inch.  They 
are  formed  with  a  loop  at  each  end,  fastened  around  a  crup- 
per-shaped piece  of  cast  iron,  that  forms  an  eye  to  connect 
the  rope  with  the  hook  of  the  stirrup  afiixed  to  the  roadway- 
bearers,  and  to  a  saddle-piece  of  wrought  iron,  for  each  rope, 
that  rests  on  the  two  main  cables. 

The  roadway-bearers  are  of  timber,  being  deeper  in  the 
centre  than  at  the  two  ends,  the  top  surface  being  curved  to 
conform  to  a  slight  transverse  curvature  given  to  the  surface 
of  the  carriage-way ;  they  are  placed  about  5  feet  between 
their  centre  lines,  every  fourth  one  projecting  about  3  feet 
beyond  the  ends  of  the  others,  to  receive  an  oblique  wrought- 
iron  stay  to  maintain  the  parapet  in  its  vertical  position.  The 
carriage-way,  which  is  about  15J  feet  wide,  is  formed  of  two 
thicknesses  of  plank.  The  foot-paths,  which  are  6  feet  wide, 
are  raised  above  the  surface  of  the  carriage-way,  and  rest 
upon  longitudinal  beams  of  large  dimensions,  the  inner  one  of 
which  is  firmly  secured  to  the  roadway-bearers  by  stirrups 
which  embrace  them,  and  the  exterior  one  is  fastened  to  the 
same  pieces  by  long  screw-bolts,  which  pass  through  the  top 
rail  of  the  parapet.  The  roadway  has  a  slight  curvature  from 
the  centre  to  the  two  extremities,  along  the  axis,  the  centre 
point  being  from  18  inches  to  about  3  ieet  higher  than  the 
ends,  according  to  the  variations  of  temperature.    The  main 


SUSPENSION  BRIDGES. 


371 


cables  at  the  centre  are  brought  down  nearly  in  contact  with 
the  roadway-timbers. 

The  parapet  is  an  open-built  beam,  consisting  of  a  top  rail, 
the  bottom  rail  being  the  longitudinal  exterior  beam  of  the 
footpath,  and  of  diagonal  pieces  which  are  mortised  into  the 
two  rails ;  the  whole  being  secured  by  the  iron  bolts  that 
pass  through  the  roadway-bearers  and  the  top  rail.  This 
combination  of  the  parapet  with  the  inclination  towards  the 
axis  of  the  ro^idway  given  to  the  suspending-ropes,  gives  great 
stiffness  to  the  roadway  and  counteracts  both  lateral  oscilla- 
tions and  longitudinal  undulations. 

The  piers  consist  of  two  pillars  of  solid  masonry,  about  66 
feet  high  above  the  level  of  the  roadway,  which  are  united,  at 
about  33  feet  above  the  same  level,  by  a  full  centre  arch, 
having  a  span  of  nearly  20  feet,  and  which  forms  the  top  of 
the  gateway  leading  to  the  bridge. 

675.  Hungerford  and  Lambeth  Bridge,  erected  over  the 
Thames,  upon  the  plans  of  Mr.  Brunei. 

This  bridge,  designed  for  foot-passengers  only,  has  the 
widest  span  of  any  chain  bridge  erected  up  to  this  period. 

Span   676 J  feet. 

Yersed  sine   50  " 

The  main  chains  are  4  in  number,  two  being  placed .  on 
each  side  of  the  bridge,  one  above  the  other.  These  chains 
are  formed  entirely  of  long  links  of  flat  bars ;  the  links  near 
the  centre  of  the  curve  having  alternately  ten  and  eleven  bars 
in  each,  and  those  near  the  piers  alternate^  eleven  and  twelve 
bars.  The  bars  are  24  feet  long,  7  inches  in  depth,  and  1  inch 
thick.  They  are  connected  by  coupling-bolts,  4f  inches  in 
diameter,  which  are  secured  at  each  end  by  cast-iron  nuts,  8 
inches  in  diameter,  and  2f  inches  thick.  The  extremity  of 
each  chain  is  connected  with  a  cast-iron  saddle-piece,  by  bolts 
which  pass  through  the  vertical  ribs  of  the  saddle-piece,  of 
which  there  are  15.  The  bottom  of  the  saddle  rests  on  50 
friction-rollers,  which  are  laid  on  a  firm  horizontal  bed  of  cast- 
iron.  The  saddle  can  move  18  inches  horizontally,  either  way 
from  the  centre,  and  thus  compensate  for  any  inequality  of 
strain  on  the  main  chains,  either  from  a  load,  or  from  vari- 
ations of  temperature. 

The  side  main-chains  are  attached  in  like  manner  to  the  sad- 
dle, and  anchored  at  the  other  extremity  in  an  abutment  of 
brickwork.  The  anchorage  (Fig.  193)  is  arranged  by  passing 
the  chains  through  a  strong  cast-iron  plate,  and  securing  the 
ends  of  the  bars  by  keys.    The  anchoring-plate  is  retained  in 


S72 


CIVIL  ENGINEERING. 


its  place  by  two  strong  cast-iron  beams,  against  which  the 
strain  upon  the  plate  is  thrown. 


Pig.  193 — Shows  the  manner  in  which  the 
Bide  main-chains  are  anchored. 

A,  inclined  shaft  for  the  chains  leading  to 
the  arched  chamber  B  of  the  anchorage. 

a,  a,  two  main-chains,  passed  through  the 
cast-iron  holding-plate  6  and  fastened  be- 
hind it  by  keys. 

c,  c,  cross  sections  of  the  cast-iron  girders 
which  retain  b. 


The  suspending-rods  (Fig.  194)  are  connected  with  both  the 


Fig.  194— Shows  an  eleva- 
tion M  and  cross  section 
N  of  the  connection  be- 
tween the  main-chains 
and  suspending-rods. 

a,  a,  upper  main-chain. 

6,  6,  joint  of  lower  main- 
chain. 

c,  suspending-rod  with  a 
forked  head  to  receive  the 
plate  d,  hung  by  stirrup- 
straps  e  and/,  respective- 
ly, to  the  coupling-bolt  of 
the  links  and  to  the  two 
bolts  g,  fastened  to  the  sad- 
dle h  on  top  of  the  upper 
main-chain. 


upper  and  lower  main-chains  ;  to  the  upper  by  a  saddle-piece 
and  bolts,  and  to  the  coupling-bolt  of  the  lower  by  an  arrange- 
ment of  articulations,  which  allows  an  easy  play  to  the  rods  ; 
at  the  bottom  (Fig.  195)  they  are  connected  by  a  joint  with  a 
bolt  that  fastens  nrmly  the  roadway-timbers. 

The  roadway-timbers  consist  of  a  strong  longitudinal  bottom 
beam,  upon  which  the  roadway-bearers  are  notched  ;  these  last 
pieces  are  in  pairs,  the  two  being  so  far  apart  that  the  bolts  con- 
necting with  the  suspending-rods  by  a  forked  head  can  pass  be- 
tween them  ;  the  flooring-plank  is  laid  upon  the  i-oadway-bear- 
ers  ;  and  a  top  longitudinal  beam,  which  forms  the  bottom  rail 
of  the  parapet,  is  secured  to  the  bottom  beam  by  the  con- 
necting bolt.    Wrought-iron  diagonal  ties  are  placed  horizon- 


SUSPENSION  BRIDGES. 


373 


tally  below  the  flooring,  to  brace  the  whole  of  the  timbers  be- 
neath. 


Fig.  195— Shows  an  elevation  of  the  roadway-timbers, 
a,  bottom  longitudinal  beam. 
6,  6,  roadway-bearers  in  pairs. 

c,  platform. 

d,  top  longitudinal  beam  forming  the  bottom  rail  of  the  para- 
pet, 

e,  bolt,  with  a  forked  head  to  receive  the  end  of  the  suspending- 
rod,  which  is  keyed  beneath  and  secures  the  beams,  etc. 

g,  wrought-iron  horizontal  diagonal  ties. 


The  roadway  is  14  feet  wide.  It  slopes  from  the  centre 
point  along  the  axis  to  the  extremities,  being  4  feet  higher  in 
the  centre  than  at  the  two  last  points. 

The  piers  are  in  the  form  of  towers,  resembling  the  Italian 
belfry.  They  are  of  brick,  80  feet  high,  and  so  constructed 
and  combined  with  the  top  saddles,  that  they  have  to  siistain 
no  other  strain  than  the  vertical  pressm-e  from  the  main-chains. 

The  "whole  weight  of  the  structure,  with  an  additional  load 
of  100  lbs.  per  square  foot  of  the  roadway,  would  throw  about 
1,000  tons  on  each  pier.  The  tension  on  the  chains  from  this 
load  is  calculated  at  about  1,480  tons  ;  while  the  strain  which 
they  can  bear  without  impairing  their  strength  is  about  5,000 
tons. 

676.  Monongahela  Wire  Bridge.  This  bridge,  erected 
at  Pittsburgh,  Penn.,  upon  plans,  and  under  the  superintend- 
ence of  the  late  Mr.  Poebling,  has  8  bays,  varying  between 
188  and  190  feet  in  width.  It  is  one  of  the  more  recent  of 
these  structures  in  the  United  States. 

The  roadway  of  each  bay  is  supported  by  two  wire  cables, 
of  4^  inches  in  diameter,  and  by  diagonal  stays  of  wire  rope, 
attached  to  the  same  point  of  suspension  as  the  cables,  and 
connecting  with  different  points  of  the  roadway-timbers. 
The  ends  of  the  cables  of  each  bay  are  attached  to  pendulum- 
bars,  by  means  of  two  oblique  arms,  which  are  united  by 
joints  to  the  pendulum-bars.  These  bars  are  suspended  from 
the  top  of  4  cast-iron  columns,  inclining  inwards  at  top, 
which  are  there  firmly  united  to  each  other ;  and,  at  bottom, 
anchored  to  the  top  of  a  stone  pier  built  up  to  the  level  of 
the  roadw^ay  timbers.  The  side  columns  of  each  frame 
are  connected  throughout  by  an  open  lozenge-work  of  cast 


374 


CIVIL  ENGINEERING. 


iron.  The  front  columns  have  a  like  connection,  leaving  a 
sufficient  height  of  passage-way  for  foot-passengers. 

The  framework  of  4  columns  on  each  side  is  firmly  con- 
nected at  the  top  by  cast-iron  beams,  in  the  form  of  an  entab- 
lature. A  carriage-way  is  left  between  the  two  frames,  and  a 
footpath  between  the  two  columns  forming  the  fronts  of  each 
frame. 

The  points  of  suspension  of  the  cables  are  over  the  centre 
line  of  the  footpaths ;  and  the  cables  are  inclined  so  far  in- 
ward that  the  centre  point  of  the  curve  is  attached  just  out- 
side of  the  carriage-way.  The  suspending-ropes  have  a  like 
inward  inclination,  the  object  in  both  cases  being  to  add  stiff- 
ness to  the  system,  and  diminish  lateral  oscillations. 

The  roadway  consists  of  a  carriage-way  22  feet  wide,  and 
two  footpaths  each  5  feet  wide.  The  roadway-bearers  aie 
transversal  beams  in  pairs,  35  feet  long,  15  inches  deep,  and 
4^  inches  wide.  They  are  attached  to  the  suspending-ropes. 
The  flooring  consists  of  2f-inch  plank,  laid  longitudinally 
over  the  entire  roadway-surface;  and  of  a  second  thickness  of 
2i-inch  oak  plank  laid  transversely  over  the  carriage-way. 

The  parapet,  which  is  on  the  principle  of  Town's  lattice, 
extends  so  far  below  the  roadway-bearers  that  they  rest  and 
are  notched  on  the  lowest  chord  of  the  lattice.  A  second 
chord  embraces  them  on  top,  and  finally  a  third  chord  com- 
pletes the  lattice  at  the  top.  The  object  of  adopting  this  form 
of  parapet  was  to  increase  the  resistance  of  the  roadway  to 
undulations. 

677.  Niagara  Railroad  and  Highway  Suspension  Bridge. 

This  remarkable  structure,  like  the  Aqueduct  suspension 
bridge  at  Pittsburgh,  was  constructed  by  Roebling ;  and  for 
boldness  of  plan,  and  skill  in  the  execution  of  its  details, 
is  every  way  worthy  of  the  professional  ability  of  this  distin- 
guished engineer. 

Designed  to  afford  a  passage-way  over  the  Niagara  river, 
both  for  railroad  and  common  road  traffic,  it  consists  essen- 
tially of  two  platforms  (Fig.  196),  one  above  the  other,  and 
about  fifteen  feet  apart ;  the  upper  serving  as  the  railroad 
track,  and  the  lower  for  ordinary  vehicles  ;  the  two  being  con- 
nected by  a  lattice  girder  on  each  side  ;  and  the  whole  bridge- 
frame  being  suspended  from  four  main  wire  cables,  two  of 
which  are  connected  with  the  upper  platform,  and  two  with 
the  lower,  by  suspension-rods  and  wire  ropes  attached  to  the 
road  way -bearers,  or  joists  of  the  platforms. 

Each  platform  consists  of  a  series  of  roadway-bearei^s  in 
pairs ;  the  lower  covered  by  two  thicknesses  of  flooring-plank, 


SUSPENSION  BRIDGES. 


375 


the  upper  by  one  thickness  ;  the  portion  of  the  latter  imme- 
diately under  the  railroad  track  having  a  thickness  of  four 
inches,  and  the  remainder  on  each  side  but  two  inches. 


Fig.  196 — Cross  section  of  Niagara  Bridge. 

A,  railway  track  and  beams.  c,  lower  main  cables. 

B,  lower  platform  for  common  road.  c',  upper  main  cables. 

C,  Diagonal  truss.  D,  suspension  ropes. 

D,  parapet.  E,  wrought-iron  braces. 

A,  lower  roadway  bearers.  F,  wooden  braces. 

a',  upper  roadway  bearers.  g,  beams  of  longitudinal  railway  bearers. 

B,  lower  flooring.  H,  longitudinal  braces  between  roadway  bearers. 
b',  upper  flooring.  N,  horizontal  rail  between  posts. 


The  lattice-girders  consist  of  vertical  posts  in  pairs,  the 
ends  of  which  are  clamped  between  the  roadway- bearers  ;• 
and  of  diagonal  wrought-iron  rods  with  screws  at  each  end, 
which  pass  through  cast-iron  plates  fastened  above  the  road- 
way-bearers of  the  upper  platform,  and  below  those  of  the 
lower,  and  are  brought  to  a  proper  bearing  by  nuts  on  each 
end.  A  horizontal  rail  of  timber  is  placed  between  the  posts 
of  the  lattice  at  their  middle  points  to  prevent  flexure. 


376 


CIVIL  ENGINEEEING. 


Fig.  197 — Side  elevation  of  Niagara  Bridge, 

A',  A',  ends  of  roadway  bearers. 

.D,  parapet. 

M,  ports  in  pairs. 

N,  rail  between  posts. 

T,  diagonal  iron  brace  rods. 

The  roadway-bearers  and  flooring  of  the  upper  platfoma 
are  solidly  clamped  between  four  solid  built  beams  or  gird- 
ers; two  above  the  flooring,  which  rest  on  cross  supports; 
and  two,  corresponding  to  those  above,  below  the  roadway- 
bearers  ;  the  upper  and  lower  corresponding  beams,  with 
longitudinal  braces  in  pairs  between  the  roadway -bearers  and 
resting  on  the  lower  beams,  being  firmly  connected  by  screw- 
bolts.    The  rails  are  laid  upon  the  top  beams. 

A  strong  parapet,  on  the  plan  of  Howe's  truss,  is  placed  on 
each  side  of  the  upper  platform. 

Wrought-iron  and  wooden  braces  connect  the  posts  and  the 
two  platforms. 

The  piers  (Fig.  198)  consist  of  four  obelisk-shaped  pillars, 
which  are  sixty  feet  high  ;  the  base  of  each  being  a  square  of 
fifteen  feet  sides ;  and  the  top  one  of  eight  feet  sides.  The 
pedestal  of  each  pillar  is  a  square  of  about  seventeen  feet 
side  at  top,  and  having  a  batir  of  one  foot  vertically  to  one 
horizontally,  or      on  each  of  its  faces.    The  height  of  the 


SUSPENSION  BRIDGES. 


377 


Fig.  19S— End  elevation  of  piers  and  con- 
necting arch  of  bridge. 

A,  shaft  of  the  pier. 

B,  pedestal. 

C,  connecting  arch, 

D,  arched  way  for  common  road. 


pedestals  on  the  United  States  side  of  the  river  being  twenty- 
eight  feet,  and  on  the  Canadian  side  eighteen  feet.  An  arch- 
way below  the  level  of  the  railroad  connects  the  two  pedestals. 

The  main  cables  pass  over  saddles  placed  on  rollers,  on 
the  tops  of  the  piers,  and  they  are  fastened  at  their  ends 
(Fig.  199)  to  chains  formed  of  links  of  wrought-iron  bars, 
which,  passing  through  abutments  of  masonry,  and  down  into 
shafts  made  into  the  solid  rock  below,  are  there  each  firmly 
attached  to  an  anchoring-plate  of  cast  iron. 

Besides  the  usual  suspending-rods  of  the  bridge,  a  number 
of  wire  ropes,  termed  over-Jloor  stays^  connect  the  portions  of 
the  upper  platform  adjacent  to  the  piers  with  the  saddles  at 
the  top  of  the  piers ;  and  the  lower  platform  is  in  like  manner 
connected  with  the  rocky  banks  of  the  river  by  a  number  of 
like  stays.  The  object  of  both  being  to  resist  the  action  of 
high  winds  upon  the  platform,  and  to  give  the  bridge  more 
rigidity. 

Each  of  the  main  cables  is  formed  of  seven  smaller  ones  or 
strands.  The  whole  bound  together  in  the  usual  manner  by 
a  wire  wrapping.  Each  strand  contains  520  wires  in  its 
cross-section,  sixty  of  which  make  an  area  of  one  square  inch. 

The  main  cables  to  which  the  roadway-bearers  of  the  upper 
platform  are  attached  are  deflected  laterally  towards  the 
axis  of  the  bridge,  and  thus  limit  the  range  of  lateral  oscilla- 
tions. This  provision,  the  lattice  structure  of  the  sides  and 
the  parapet,  the  over  and  under  floor  stays,  the  deep  longitu- 
dinal girders  of  the  railway  track,  the  slight  camber  or  longi- 


378 


CIVIL  ENGINEERING. 


Fig.  199 — Side  view  of  anchor-chain, 

A,  masonry  of  buttress. 

B,  natural  rock  bed. 

C,  shaft  and  masonry  for  chains. 

D,  anchoring-plate. 


tudinal  curvature  from  the  ends  of  the  bridge  to  the  centre, 
and  its  own  weight,  give  to  the  whole  structure  that  degree 
of  rigidity  and  stability  which  are  its  marked  characteristics, 
as  contrasted  with  suspension  bridges  usually. 

Some  of  the  principal  dimensions  of  the  means  of  suspen- 
sion are  given  in  the  following  statement : 

Span  of  both  cables  between  axis  of  piers,  821^  feet. 

Yersed  sine  of  cables  of  lower  platform,  64  feet. 

Versed  sine  of  cables  of  upper  platform,  54  feet. 

Diameter  of  each  cable,  10  inches. 

Area  of  cross-section  of  each  cable,  60.4  square  inches. 

Area  of  cross-section  of  upper  links  of  anchor-chains,  372 
square  inches. 

Ultimate  strength  of  anchor-chains,  11,904  tons. 

Number  of  wires  in  the  four  cables,  14,560. 

Average  strength  of  one  wire,  1,648  lbs. 

Ultimate  strength  of  the  four  cables,  12,000  tons. 

Permanent  weight  borne  by  the  cables,  1,000  tons. 

Length  of  anchor-chains,  66  feet. 

Length  of  upper  cables,  1,261  feet. 


STJSPENSION  BRIDGES. 


379 


Length  of  lower  cables,  1,193  feet. 
ISTumber  of  suspenders,  624. 
J^umber  of  over-floor  stays,  64. 
Number  of  under-floor  stays,  56. 
Length  of  platforms  between  piers,  800  feet. 
Height  of  railway  track  above  middle  stage  of  water,  245 
feet. 

678.  East  River  Bridge.  The  East  Eiver  Bridge,  which  is 
now  in  process  of  erection,  will,  when  completed,  be  the 
longest  span  suspension  bridge  which  has  been  erected  up  to 
this  date.  It  will  form  a  suspended  highway  connecting 
New  York  and  Brooklyn  cities.  The  terminus  in  New  York 
city  will  be  opposite  City  Hall,  in  Chatham  street ;  and  in 
Brooklyn  in  the  square  bounded  by  Fulton,  Sands,  Washing- 
ton, and  Prospect  streets.  Its  total  length  will  be  5,989  feet. 
The  central  span  will  cross  the  river  without  impeding  navi- 
gation, in  a  single  span  of  1,595  feet  6  inches  from  centre  to 
centre  of  tower. 

On  each  side  of  the  central  opening  on  the  land  sides  there 
will  be  spans  supported  by  the  land  cables  of  930  feet  each. 
The  remaining  distances,  which  form  the  approaches,  will  be 
supported  by  iron  girders  and  trusses,  and  will  rest  at  short 
intervals  upon  small  piers  of  masonry  or  iron  columns, 
located  within  the  blocks  of  buildings  which  will  be  crossed 
and  occupied.  These  pillars  will  form  part  of  the  walls 
needed  for  the  division  of  the  occupied  ground  into  stores, 
dwellings,  or  offices. 

The  grade  from  the  New  York  terminus  to  the  centre  of 
the  bridge  will  be  three  feet  and  three  inches  per  hundred 
feet,  and  the  same  on  the  Brooklyn  side  from  the  centre  of 
the  bridge  to  the  anchorage,  but  the  grade  of  the  Brook- 
lyn approach  will  be  two  feet  and  nine  inches  per  hundred 
feet. 

The  floor  of  the  bridge  will  be  85  feet  in  width  from  out 
to  out.  The  floor  is  divided  into  five  spaces  by  six  lines  of 
iron  trusses.  The  outer  spaces  will  be  in  the  clear  eigh- 
teen feet  each,  and  will  accommodate  each  two  lines  of  iron 
tramways  for  ordinary  vehicle  travel,  as  well  as  for  street  cars, 
drawn  singly  by  horses,  or  in  pairs  by  light  dummies.  The 
next  two  spaces  will  be  thirteen  feet  two  inches  wide  each, 
provided  with  an  iron  track  for  running  of  two  passenger 
trains  back  and  forward  alternately.  These  trains  will  be  at- 
tached to  an  endless  wire  rope,  propelled  by  a  stationary  en- 
gine, which  will  be  located  on  the  Brooklyn  side,  underneath 
the  floor,  the  two  tracks  being  operated  like  an  inclined  plane. 


380 


CIVIL  ENGINEERING. 


with  a  speed  of  twenty  miles  per  hour,  the  whole  transit  occu- 
pying only  five  minutes  from  terminus  to  terminus. 

The  central  or  fifth  division  of  the  bridge  floor  will  form  a 
promenade  for  foot  travel,  fifteen  feet  in  width.  It  will  be 
elevated  five  feet  above  the  roadway,  affording  a  view  over 
both  sides  of  the  bridge. 

The  roadway  will  pass  the  towers  at  an  elevation  of  119 
feet,  and  the  centre  of  the  main  span  will  be  135  feet  above 
mean  high  tide,  or  140  feet  above  mean  low  water. 

The  width  of  the  roadway,  from  outside  to  outside,  will  be 
85  feet. 

The  bridge  will  be  supported  by  four  main  cables,  each  16 
inches  in  diameter,  composed  of  galvanized  tempered  cast- 
steel  wire,  'No.  6  gauge,  having  a  strength  of  160  pounds  per 
square  inch  of  section.  There  will  also  be  104  stays  to  aid 
the  cables. 

The  total  weight  of  the  structure,  including  the  cables,  is 
estimated  to  be  5,000  tons. 

This  grand  structure  was  devised,  and  works  superintended 
till  his  death,  by  the  late  John  A.  Roebling.  It  is  now  engi- 
neered by  his  son  Col.  W.  A.  Roebling. 

YIII. 

MOVABLE  BEIDGES. 

679.  The  term  movable  bridge  is  commonly  applied  to  a 
platform  supported  by  a  framework  of  timber  or  of  cast 
iron,  by  means  of  which  a  communication  can  be  formed  or 
interrupted  at  pleasure  between  any  two  points  of  a  fixed 
bridge,  or  over  any  narrow  water-way.  These  bridges  are 
generally  denominated  draiv-hridges,  but  this  term  is  now,  for 
the  most  part,  confined  to  those  movable  bridges  which  can 
be  raised  or  lowered  by  means  of  a  horizontal  axis,  placed 
either  at  one  extremity  of  the  platform,  or  at  some  inter- 
mediate point  between  the  two  ends,  and  a  (counterpoise  which 
is  so  connected  with  the  platform  in  either  case,  that  the 
bridge  can  be  easily  manoeuvred  by  a  small  power  acting 
through  the  intermedium  of  some  suitable  mechanism  ap- 
plied to  the  counterpoise.  Tlie  term  turning  or  swinging 
bridge  is  used  when  the  bridge  is  arranged  to  turn  horizon- 
tally around  a  vertical  axis  placed  at  a  point  between  its  two 
ends,  so  that  the  parts  on  each  side  of  the  axis  balance  each 
other ;  and  the  term  rolling  bridge  is  applied  when  the  bridge, 


MOVABLE  BRIDGES. 


381 


resting  upon  rollers,  can  be  shoved  forward  or  backward  hori- 
zontally, to  open  or  interrupt  the  passage. 

To  the  above  may  be  added  another  class  of  movable 
bridges  used  for  the  same  purpose,  which  consist  of  a  plat- 
form supported  by  a  boat,  or  other  buoyant  body,  which  can 
be  placed  in  or  withdrawn  from  the  water-way  as  circum- 
stances may  require. 

680.  Draw-Bridges.  When  the  horizontal  axis  of  this 
description  of  bridge  is  placed  at  the  extremity  of  the  plat- 
form, the  bridge  is  manoeuvred  by  attaching  a  chain  to  the 
other  extremity,  which  is  connected  with  a  counterpoise  and 
a  suitable  mechanism,  by  which  the  slight  additional  power 
required  for  raising  the  bridge  can  be  applied. 

Fig.  200— Shows  the  manner 

)of  manoeuvring  a  draw- 
bridge either  by  a  framed 
lever,  or  by  a  counterpoise 
suspended  from  a  spiral 
eccentric. 
A,  abutment. 

a,  section  of  the  platform. 
6,  framed  lever, 
c,  chain  attached  to  the  ends 
of  the  lever  and  the  plat- 
form. 

a,  strut  movable  around  its 
lower  end. 

e,  bar  with  an  articulation 
at  each  end  that  confines 
the  strut  to  the  platform, 
y,  spiral  eccentric  connected 
with  the  counterpoise  g  by 
a  chain  passing  over  the 
gorge  of  the  eccentric. 

h,  chain  for  raising  the 
bridge,  one  end  of  which 
is  attached  to  the  extremity 
of  the  platform,  and  the 
other  to  the  axle  of  the 
eccentric. 

i,  fixed  pulley  over  which  the 
chain  h  is  passed. 

TO,  wheel  fixed  to  the  axle 
of  the  eccentric  for  the 
purpose  of  turning  it  by 
means  of  animal  power 
applied  to  the  endless 
chain  n. 


A  number  of  ingenious  contrivances  have  been  put  in 
practice  for  these  purposes.  They  consist  usually  either  of  a 
counterpoise  of  invariable  weight,  connected  with  additional 
animal  motive-power,  which  acts  with  constant  intensity,  but 
with  a  variable  arm  of  lever  ;  or  of  a  counterpoise  of  vari- 
able weight,  which  is  assisted  by  animal  motive-power  acting 
with  an  invariable  arm  of  lever.  In  some  cases  the  bridge  is 
worked  with  a  less  complicated  combination,  by  dispensing 


'382 


CIVIL  ENGINEERING. 


with  a  counterpoise,  and  applying  animal  motive-power,  of 
variable  intensity,  acting  with  a  constant  or  a  variable  arm  of 
lever. 

Among  the  combinations  of  the  first  kind  the  most  simple 
consists  in  placing  a  framed  lever  (Fig.  200)  revolving  on  a 
horizontal  axis  above  the  platform.  The  anterior  part  of  the 
frame  is  connected  with  the  movable  extremity  of  the  plat- 
form by  two  chains.  The  posterior  portion,  which  forms  the 
counterpoise,  has  chains  attached  to  it  by  which  the  lever  can 
be  worked  by  men. 

When  the  locality  does  not  admit  of  tliis  arrangement,  the 
chain  attached  to  the  movable  end  of  the  platform  may  be 
connected  with  a  horizontal  axle  above  the  platform,  to  which 
is  also  attached  a  fixed  eccentric  of  a  spiral  shape  (Fig.  200), 
connected  with  a  chain  that  passes  over  its  gorge  and  sustains 
a  counterpoise  of  invariable  weight.  Upon  the  same  axle  an 
ordinary  wheel  is  hung,  over  the  gorge  of  which  passes  an 
endless  chain  to  manoeuvre  the  bridge  by  animal  power. 

Fig.  201— Shows  the  ar- 
rangement of  a  draw- 
bridge with  a  variable 
counterpoise. 
A  and  B,  abutments, 
gf,  variable  counterpoise 
formed  of  a  chain  with 
flat  links,  one  end  of 
which  is  attached  to  a 
fixed  point,  and  the 
other  to  the  chain  c  at- 
tached to  the  movable 
end  of  the  platform. 
i,  fixed  puUey  over  which 
the  chain  c  passes  to 
the  small  wheel  k  fixed 
on  a  horizontal  shaft, 
to  which  is  also  attach- 
ed the  wheel  m  and 
the  endless  chain  n 
for  manoeuvring  the 
bridge. 

Of  the  combinations  of  variable  counterpoises  the  mechan- 
ism of  M.  Poncelet,  which  has  been  successfully  applied  in 
many  instances  in  France  for  the  draw-bridges  of  military 
works,  is  one  of  the  most  simple  in  its  arrangement  and  con- 
struction. The  movable  end  of  the  platform  (Fig.  201)  is 
connected  by  a  common  chain,  that  passes  over  the  gorge  of  a 
wheel  hung  upon  a  horizontal  shaft  above  the  platform,  with 
another  chain  of  variable  breadth,  formed  of  flat  bar  links, 
which  forms  the  counterpoise.  The  chain  counterpoise  is  at- 
tached at  its  other  extremity  to  a  fixed  point  in  such  a  way, 
that  when  the  platform  ascends  a  portion  of  the  weight  of 
the  chain  is  borne  by  this  fixed  point ;  and  thus  the  weight  of 


MOVABLE  BRIDGES. 


383 


the  counterpoise  decreases  as  the  platform  rises.  The  system 
is  manoeuvred  by  an  endless  chain  passed  over  the  gorge  of  a 
wheel  hung  upon  the  horizontal  shaft. 

For  light  platforms  a  counterpoise  may  be  dispensed  with, 
and  the  bridge  may  be  manoeuvred  by  connecting  the  chain 
attached  to  the  movable  end  of  the  platform  to  a  horizontal 
shaft,  which  is  turned  by  the  usual  tooth-work  combinations. 

When  the  locality  does  not  admit  of  manoeuvring  the 


Fig.  202— Shows  the  ar- 
rangement of  a  draw- 
bridge where  the  coun- 
terpoise is  formed  by 
prolonging  back  the 
platform. 

A,  abutment. 

B,  well  of  a  suitable  form 
for  manceuvring  the 
bridge. 

a,  chain-stay  to  keep  the 
platform  firm  when  the 
bridge  is  down. 


bridge  by  a  chain  connected  with  some  point  above  the 
fi-amework,  the  platform  (Fig.  202)  is  continued  back,  from 
two-thirds  to  three-fifths  its  length,  from  the  face  of  the 
abutment,  to  fbrm  a  counterpoise  for  the  platform  of  the 
bridge.  The  horizontal  axis  of  the  bridge  is  placed  near  the 
face  of  the  abutment,  and  a  well  of  a  suitable  shape  to  re- 
ceive the  posterior  portion  of  the  platform  that  forms  the 
counterpoise  is  formed  behind  the  abutment. 

The  mechanism  for  working  the  bridge  may  consist  of  a 
chain  and  capstan  below  the  platform-counterpoise,  or  of  a 
suitable  combination  of  tooth-work. 

In  bridges  of  a  single  platform,  the  movable  extremity, 
when  the  bridge  is  lowered,  rests  on  the  opposite  abutment, 
and  no  intermediate  support  will  be  required  for  the  struc- 
ture if  the  framework  be  of  sufficient  strength ;  but  when 
a  double  bridge,  consisting  of  two  platforms,  is  used,  the  plat- 
forms (Fig.  200)  should  be  supported  near  their  movable  ends, 
when  the  bridge  is  down,  by  struts  movable  around  the  joint 
by  which  they  are  connected  with  the  face  of  the  abutments. 
These  struts  are  so  connected  with  the  bridge  that  they  are 
detached  from  it  and  drawn  up  when  it  is  raised,  and  fall  back 
into  their  places,  abutting  against  blocks  near  the  movable  end 
of  the  platform,  when  the  bridge  is  down.  By  these  arrange- 
ments the  chains  for  working  the  bridge  are  relieved  from  a 


384 


CIYTL  ENGINEERING. 


portion  of  the  strain  when  the  bridge  is  down,  and  it  is  also 
rendered  more  firm. 

Wlien  the  counterpoise  is  formed  by  the  rear  part  of  the 
platform,  additional  security  may  be  given  to  the  bridge  when 
down  by  attaching  two  chains  beneath  the  platform,  and  se- 


In  some  cases  a  heavy  bar,  fitted  to  staples  beneath  connected 
with  the  timbers  of  the  platform,  is  used  for  the  same  pur- 
pose. 

In  double  bridges  the  two  platforms  when  lowered  should 
abut  against  each  other,  giving  a  slight  elevation  to  the  cen- 
tre of  the  bridge.  This  not  only  gives  greater  stiffness,  but 
is  favorable  to  detaching  the  platforms  when  the  bridge  is  to 
be  raised. 

For  draw,  and  every  kind  of  movable  bridge,  temporary 
barriers  should  be  erected  on  each  side  at  the  entrance  upon 
the  bridge,  to  prevent  accidents  by  persons  attempting  to 
cross  the  bridge  before  it  is  properly  secured  when  lowered. 

681.  Turning-bridges.  These  bridges  revolve  horizontally 
upon  a  vertical  shaft  or  gudgeon  below  the  platform,  which 
is  usually  thrown  far  enough  back  from  the  face  of  the  abut- 
ment to  place  the  side  of  the  bridge,  when  brought  round, 
just  within  this  face.  The  weights  of  the  parts  of  the  bridge 
around  the  shaft  should  balance  each  other. 


Fig.  203— Represents  the  arrangement  of  a  turning-bridge. 

a,  platform  of  the  bridge. 

b,  vertical  posts  to  which  the  iron  stays  n  n  are  attached. 

c,  vertical  shaft  or  gudgeon  on  which  the  bridge  turns. 
o  o,  conical  rollers. 


To  support  and  manoeuvre  the  bridge  (Fig.  203)  a  circular 
ring  of  iron,  or  roller-way,  of  less  diameter  than  the  breadth 
of  the  bridge,  and  concentric  "with  the  vertical  shaft,  is  firmly 
imbedded  in  masonry.  Fixed  rollers,  in  the  shape  of  trun- 
cated cones,  are  attached  at  equal  distances  apart  to  the  frame- 
work of  the  platform  beneath,  and  rest  upon  the  roller- way. 


curing 


bottom  of  the  well. 


swmG  BEroGES. 


385 


The  bridge  is  worked  bj  a  suitably  arranged  tooth-work,  or 
by  a  chain  and  capstan.  In  some  cases  cast-iron  balls,  rsst- 
ing  on  a  grooved  roller-way,  and  fitting  into  one  of  corre- 
sponding shape  fixed  beneath  the  platform,  have  been  used 
for  manoeuvring  the  bridge. 

The  ends  of  the  bridge  are  cut  in  the  shape  of  circular  arcs 
to  fit  recesses  of  a  corresponding  form  in  the  abutments,  so 
arranged  as  not  to  impede  the  play  of  the  bridge. 

In  double-turning  bridges  the  two  ends  of  the  platforms 
which  come  together  should  be  of  a  curved  shape.  The  plat- 
forms should  be  sustained  from  beneath  by  struts,  like  those 
used  for  draw-bridges,  which  can  be  detached  and  drawn  into 
recesses  when  the  passage  is  interrupted ;  or  else  they  may 
be  arranged  with  a  ball-and-socket  joint  at  their  lower  ex- 
tremity, so  as  to  be  brought  round  with  the  bridge.  For  the 
purpose  of  giving  additional  strength  and  security  to  the 
bridge,  iron  stays  are,  in  some  cases,  attached  on  each  side  of 
the  platform  near  the  extremities,  and  connected  with  verti- 
cal posts  placed  in  a  line  with  the  vertical  shaft. 

Turning-bridges  may  be  made  either  of  timber  or  of  cast 
iron ;  the  latter  material  is  the  more  suitable,  as  admitting  of 
more  accuracy  of  workmanship,  and  not  being  liable  to  the 
derangements  caused  by  the  shrinking  or  warping  of  frame- 
work of  timber. 

632.  Swing  Bridge  at  Providence,  R.  I.  The  details  of 
this  bridge  are  worthy  of  special  study.  An  account  of  it 
is  published  in  the  London  Engineering  for  March  21st, 
1873.  Fig.  204  is  an  elevation  of  the  bridge,  and  the  right- 
hand  half  of  Fig.  205  is  a  plan  of  the  truss  work  under  the 
roadway,  and  the  left-hand  half  the  plan  of  the  roadway  and 
truss  work.  Fig.  206  is  a  section  of  the  turn-table  for  sup- 
porting the  bridge.  An  essential  part  is  the  four  compound 
radial  arms,  G  G,  F  F,  Fig.  206,  the  lower  parts  of  which 
are  of  cast-iron  compression  members,  and  the  upper  parts 
of  two  wrought-iron  rods  each. 

The  whole  structure  rests  upon  a  nest  of  conical  rollers,  1 1 
(Fig.  206),  upon  which  it  turns  as  it  moves  about.  There  are 
several  small  wheels  5,  5,  5,  which  are  under  the  turn-table, 
and  serve  only  to  steady  it  in  case  it  tends  to  tip  in  any  di- 
rection. 

The  strains  on  the  several  members  were  computed  under 
three  hypotheses,  viz. :  1st.  The  strains  due  to  the  weight  of 
the  truss  only  when  the  draw  was  open.  These  strains  were 
assumed  to  be  the  same  as  when  it  was  closed  and  unloaded, 
for  no  part  of  the  weight  of  the  bridge  was  supposed  to  be 
25 


388 


CIVIL  ENGINEERING. 


supported  at  its  ends,  although  the  ends  were  pinned  to  keep 
them  from  rising  when  only  one  part  was  loaded.  2d.  One 
half  was  supposed  to  be  loaded  while  the  other  end  was  held 
down  by  the  pin  ;  and  3d.  The  bridge  was  supposed  to  be 
loaded  uniformly  throughout. 

The  call  for  proposals  specified  that  the  rolling  load  should 
be  3,200  lbs.  per  lineal  foot  of  the  bridge,  and  that  the 
wrought  iron  should  not  be  strained  in  tension  to  exceed 
12,000  lbs.  per  square  inch,  or  in  compression  8,000  lbs.  per 
square  inch.  The  following  tables  give  the  results  of  the 
original  computations  for  the  strains  and  the  dimensions  of 
the  pieces  used.  The  engineer,  Charles  McDonald,  of  New 
York  City,  states  that  a  review  of  the  computations  after  the 
structure  was  completed,  confirmed  the  general  results,  al- 
though in  some  cases  the  actual  strains  exceed  those  previ- 
ously determined  by  a  small  amount.  Although  the  analysis 
shows  (see  Table  II.),  that  there  is  compression  on  the  fourth 
and  fifth  bay  of  the  upper  chord,  yet  there  is  no  tendency  to 
a  strain  on  the  counter-diagonals  in  those  panels.  The  incli- 
nation of  the  upper  chord  acts  as  a  brace  and  thus  prevents 
any  strain  in  the  direction  of  the  counter-tie  in  those  panels. 

Table  No.  I. — Showing  Total  Strains  on  Parts  uchen  the  Bridge  is  Open^ 

but  Unloaded. 


(The  sign  plits  is  for  compression  and  minus  for  tension. ) 


Number  of 
Bay. 

Top  Chord. 

Bottom  Chord. 

Verticals. 

Diagonals. 

Counter- 
ties. 

lb. 

lb. 

lb. 

lb. 

lb. 

End 

nil. 

+  6,073 

nil 

-  8,427 

ml 

2 

-  6,223 

+  19,577 

+  4,407 

-  20,900 

a 

3 

-  19,941 

+  37,735 

+  13,532 

-  30,868 

(( 

4 

-  38,203 

+  59,688 

+  22,565 

-  40,800 

5 

-  60,280 

+  85,759 

+  32,227 

-  51,340 

6 

-  86,300 

+  116,189 

+  42,702 

-  62,600 

7 

-116,000 

+  151,625 

+  55,047 

-  75.375 

8 

-151,860 

+  193,249 

+  68,463 

-  90;350 

9 

-193,400 

+  242,624 

+  84,090 

-107,637 

Centre 

-242,024 

+  242,624 

+  98,625 

nil  ' 

SWING  BEIDGES. 


389 


Table  No.  II. — Showing  Total  Strains  on  Parts  with  Bndge  Closed  and 
me-half  fuUy  Loaded,  the  Unloaded  end  being  Latched. 


Number  of 
Bay, 

Top  Chord. 

Bottom  Chord. 

Verticals. 

Diagonals. 

Counter" 
ties. 

lb. 

lb. 

lb. 

lb. 

lb. 

Loaded  end 

+  69,500 

nil 

+  64,500 

nil 

—  81,080 

3 

+  83,610 

—  67,480 

+  21,500 

nil 

—  27,000 

8 

+  83,110 

—  69,800 

nil 

nil 

nil 

4 

+  69,977 

—  41,600 

+  17,718 

—  52,249 

5 

+  42,000 

nil 

+  40,365 

—  81,910 

6 

nil 

+  53,800 

+  64,500 

—110,674 

7 

—  54,000 

+  120,337 

+  92,690 

—141,580 

8 

—120,520 

+  201,326 

+  123,440 

—175,770 

9 

—201,480 

+  299,537 

+  158,187 

—214,100 

Centre 

-299,537 

+  304,868 

j  +193,500) 
(  +loU,01U  ) 

-  60,560 

9 

-249,140 

+  304,868 

+  96,480 

-121,916 

8 

-201,954 

+  248,943 

+  79,520 

-103,670 

7 

-161,500 

+  201,637 

+  65,190 

-  86,618 

6 

-126,120 

+  160,915 

+  51,800 

-  73,141 

5 

-  95,240 

+  125,360 

+  41,036 

-  61,061 

4 

-  67,713 

+  94,300 

+  30,984 

-  51,000 

3 

-  42,962 

+  66,778 

+  22,400 

-  41,955 

nil 

2 

-  20,600 

+  42,178 

+  14,500 

-  34,240 

nil 

Unloaded  end 

ml 

+  20,098 

nil 

-  27,752 

nil 

Table  No.  111.— Showing  Total  Strains  on  Parts  with  Bridge  closed  and 
fully  Loaded. 


Number  of 
Bay. 

Top  Chord. 

Bottom  Chord. 

Verticals. 

Diagonals. 

Counter- 
ties. 

lb. 

lb. 

lb. 

lb. 

lb. 

End 

+  52,130 

nU 

+  48,425 

nil 

-  60,810 

-  13,500 

2 

+  55,774 

-  50,611 

+  10,500 

nil 

3 

+  55,100 

-  34,940 

nn 

-  26,727 

nil 

4 

+  35,430 

nU 

+  24,157 

-  64,732 

5 

nil 

-  47,644 

+  48,425 

-  94,383 

6 

-  47,930 

+  107,600 

+  74,621 

-123,340 

7 

-108,000 

+  180,500 

+  104,638 

-155,073 

8 

-180,779 

+  268,200 

+  136,540 

-190,300 

9 

-268,400 

+  374,421 

+  172,803 
+  209,625 

-231,560 

Centre 

-374,421 

+  374,420 

nil 

390 


CIVIL  ENGINEERING. 


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0) 


AQUEDUCT  BEIDGES. 


391 


683.  Rolling-'bridges.  These  bridges  are  placed  upon 
fixed  rollers,  so  that  they  can  be  moved  forward  or  backward, 
to  interrupt  or  open  the  communication  across  the  water- 
way. The  part  of  the  bridge  that  rests  upon  the  rollers, 
when  the  passage  is  closed,  must  form  a  counterpoise  to  the 
other.  The  mechanism  usually  employed  for  manoeuvring 
these  bridges  consists  of  tooth-work,  and  may  be  so^  arranged 
that  it  can  be  worked  by  one  or  more  persons  standing  on  the 
bridge.  Instead  of  fixed  rollers  turning  on  axles,  iron  balls, 
resting  in  a  grooved  roller- way,  may  be  used,  a  similar  roller- 
way  being  afiixed  to  the  framework  beneath. 

684.  Boat-bridge.  A  movable  bridge  of  this  kind  may 
be  made  by  placing  a  platform  to  form  a  roadway  upon  a 
boat,  or  a  water-tight  box  of  a  suitable  shape.  This  bridge 
is  placed  in,  or  withdrawn  from  the  water-way,  as  circum- 
stances may  require,  a  suitable  recess  or  mooring  being  ar- 
ranged for  it  near  the  water-way  when  it  is  left  open. 

A  bridge  of  this  character  cannot  be  conveniently  used  in 
tidal  waters,  except  at  certain  stages  of  the  water.  It  may 
be  employed  with  advantage  on  canals  in  positions  where  a 
fixed  bridge  could  not  be  placed. 

IX. 

AQUEDUCT-  BEIDGES. 

685.  In  aqueducts  and  aqueduct-bridges  of  masonry,  for 
supplying  reservoirs  for  the  wants  of  a  city,  or  for  any  other 
purpose,  the  volume  of  water  conveyed  being,  generally 
speaking,  small,  the  structure  will  present  no  peculiar  difii- 
culties  beyond  affording  a  water-tight  channel.  This  may  be 
made  either  of  masonry,  or  of  cast-iron  pipes,  according  to 
the  quantity  of  water  to  be  delivered.  If  formed  of  masonry, 
the  sides  and  bottom  of  the  channel  should  be  laid  in  the 
most  careful  manner  with  hydraulic  cement,  and  the  surface 
in  contact  with  the  water  should  receive  a  coating  of  the 
same  material,  particularly  if  the  stone  or  brick  used  be  of  a 
porous  nature.  This  part  of  the  structure  should  riot  be 
commenced  until  the  arches  have  been  un centred  and  the 
heavier  parts  of  the  structure  have  been  carried  up  and  have 
had  time  to  settle.  The  interior  spandrel-filling,  to  the  level 
of  the  masonry  which  forms  the  bottom  of  the  water-way, 
may  either  be  formed  of  solid  material,  of  good  rubble  laid 
in  hydraulic  cement,  or  of  beton  well  settled  in  layers  ;  or  a 
system  of  interior  walls,  like  those  used  in  common  bridges 


392 


CIVIL  ENGINEERING. 


for  the  support  of  the  roadway,  may  be  used  in  this  case  for 
the  masonry  of  the  water-way  to  rest  on. 

686.  In  canal  aqueduct-bridges  of  masonry,  as  the  Yohime 
of  water  required  for  the  purposes  of  navigation  is  much 
greater  than  in  the  case  of  ordinary  aqueducts,  and  as  the 
structure  has  to  be  traversed  by  horses,  every  precaution 
should  be  taken  to  procure  great  solidity,  and  secure  the 
work  from  accidents. 

Segment  arches  of  medium  span  will  generally  be  found 
most  suitable  for  works  of  this  character.  The  section  of 
the  water-way  is  generally  of  a  trapezoidal  form,  the  bottom 
line  being  horizontal,  and  the  two  sides  receiving  a  slight 
batir ;  its  dimensions  are  usually  restricted  to  allow  the  pas- 
sage of  a  single  boat  at  a  time.  On  one  side  of  the  water- 
way a  horse  or  tow-path  is  placed,  and  on  the  other  a  narrow 
footpath.  The  water-way  should  be  faced  with  a  hard  cut- 
stone  masonry,  well  bonded  to  secure  it  from  damage  from 
the  passage  of  the  boats.  The  space  between  the  facing  of 
the  water-way,  termed  the  trunk  of  the  aqueduct,  and  the 
head-walls,  is  filled  in  with  solid  material,  either  of  rubble  or 
of  beton. 

A  parapet- wall  of  the  ordinary  form  and  dimensions  sur- 
mounts the  tow  and  foot  paths. 

The  approach  to  an  aqueduct-bridge  from  a  canal  is  made 
by  gradually  increasing  the  width  of  the  trunk  between  the 
wings,  which,  for  this  purpose,  usually  receives  a  curved 
shape,  and  narrowing  the  water-way  of  the  canal  so  as  to 
form  a  convenient  access  to  the  aqueduct.  Great  care  should 
be  taken  to  form  a  perfectly  water-tight  junction  between 
the  two  works. 

687.  When  cast  iron  or  timber  is  used  for  the  trunk  of  an 
aqueduct-bridge,  the  abutments  and  piers  should  be  built  of 
stone.  The  trunk,  which,  if  of  cast  iron,  is  formed  of  plates 
with  flanches  to  connect  them,  or,  if  of  timber,  consists  of 
one  or  two  thicknesses  of  plank  supported  on  the  outside  by 
a  framing  of  scantling,  may  be  supported  by  a  bridge-frame 
of  cast  iron,  or  of  timber,  or  be  suspended  from  chains  or 
wire  cables. 

The  tow-path  may  be  placed  either  within  the  water-way, 
or,  as  is  most  usually  done,  without.  It  generally  consists  of 
a  simple  flooring  of  plank  laid  on  cross-joists  supported  from 
beneath  by  suitably-arranged  framework. 


EOOFS, 


393 


CHAPTEE  yi. 

EOOFS. 

688.  A  Roof,  in  common  language,  is  the  covering  over  a 
structure,  the  chief  object  of  which  is  to  protect  the  building 
against  the  effects  of  snow  and  rain.  It  is  composed  of 
boards,  shingles,  slate,  mastic,  or  other  suitable  materials. 


Fig.  207. 

The  inclined  pieces  AC,  and  BC,  Fig.  207,  which  support 
the  roof  are  called  rafters.  When  the  roof  is  light,  the  roof 
boards  DE  are  placed  directly  upon  the  rafters,  but  when  the 
rafters  are  far  apart,  say  more  than  four  feet,  small  pieces 
5,  c,  and  called  purlins^  are  placed  across  the  rafters  for 
the  purpose  of  receiving  the  roof  proper.  AB  is  a  tie,  and 
F  and  G  represent  the  ends  of  posts.  The  frame  ABC  is 
called  a  roof  truss, 

689.  Roof  Trusses  have  a  great  variety  of  forms,  and 
differ  greatly  in  the  details  of  their  construction.  All  the 
trusses  which  have  been  discussed  in  the  preceding  pages  are 
suitable  for  this  purpose  in  many  cases.  Some  other  forms 
are  given  in  the  following  pages. 

690.  General  Data.  A  roof  truss  is  required  to  carry 
its  own  weight,  the  weight  of  the  purlins,  the  weight  of  the 


Purlin  beams  are  sometimes  placed  under  the  rafters. 


394 


CIVIL  ENGINEERING. 


roof  above  them,  the  force  of  the  wind,  the  weight  of  snow 
when  there  is  any,  and  in  some  cases  certain  local  or  concen- 
trated loads,  such  as  floors,  machinery,  and  the  like,  which 
are  suspended  from  the  roof  trusses. 

691.  The  Weight  of  Snow.  Freshly  fallen  snow  weighs 
from  five  to  twelve  lbs.  per  cubic  foot,  although  snow  which 
is  saturated  with  water  weighs  much  more.  Some  say  that 
snow  is  equivalent  to  from  to  -J-  of  its  depth  in  water, 
while  others  say  that  it  may  be  equivalent  to  J  its  depth  of 
water. 

European  engineers  consider  that  six  lbs.  per  square  foot 
is  sufiicient  for  snow,  and  eight  lbs.  for  the  pressure  of  the 
wind,  making  fourteen  lbs.  for  both.  Trautwine  thinks  that 
not  less  than  twenty  lbs.  should  be  allowed  in  the  United 
States. 

692.  The  Force  of  the  Wind.  According  to  Mr. 
Smeaton,  the  pressure  of  the  wind  directly  against  a  flat  sur- 
face in  a  hurricane  may  be  32  lbs.  per  square  foot.  Tred- 
gold  recommends  an  allowance  of  40  lbs.  per  square  foot. 
A  gauge  in  Girard  College  broke  under  a  strain  of  42  lbs.  per 
square  foot,  whilst  a  tornado  was  passing  near  by.  During 
the  severest  gale  on  record  at  Liverpool,  England,  there  was 
a  pressure  of  42  lbs.  per  square  foot  directly  upon  a  flat  sur- 
face. During  a  very  violent  gale  in  Scotland,  a  wind-gauge 
once  indicated  45  lbs.  per  square  foot.  Buildings  which  are 
more  or  less  protected  will  not  be  subjected  to  such  high 
pressures. 


Fig.  208 — Represents  a  roof  truss  for  medium  spana. 
O,  tie-beam  of  truss. 

b,  6,  principal  rafters  framed  into  tie-beam  and  the  king  post  c,  and  confined  at  thtir 

foot  by  an  iron  strap. 
d,  d,  struts. 

€,  e,  purlins  supporting  the  common  rafters/,  /. 

693.  The  truss  of  a  roof,  for  ordinary  bearing,  consists 
(Fig.  208)  of  a  horizontal  beam  termed  the  tie-bea?n,  with 
which  the  inclined  beams,  termed  the  principal  rafters^  are 
connected  by  suitable  joints.    The  principal  rafters  may 


EOOFS. 


395 


either  abut  against  each  other  at  the  top  or  ridge^  or  against 
a  king  post.  Inclined  struts  are  in  some  cases  placed  be- 
tween the  principal  rafters  and  king  post,  with  which  they 
are  connected  by  suitable  joints. 

For  wider  bearings  the  short  rafters  (Fig.  209)  abut  against 
a  straining  beam  at  the  top.  Queen  posts  connect  these  pieces 
with  the  tie-beam.  A  king  post  connects  the  straining  beam 
with  the  top  of  the  short  rafters ;  and  struts  are  placed  at 
suitable  points  between  the  rafters  and  king  and  queen  posts. 

Fig.  209— Represents  a  roof  truss  for  wide 

spans, 
a,  tie-beam. 
6,  6,  principal  rafters. 

c,  short  rafters  abutting  against  the  strain- 
ing beam  d. 
e  and/,  king  and  queen  pests  in  pairs. 
St  Qi  purlins  supporting  common  rafters  h. 

In  each  of  these  combinations  the  weight  of  the  roof 
covering  and  the  frames  is  supported  by  the  points  of  support. 
The  principal  rafters  are  subjected  to  cross  and  longitudinal 
strains*  arising  from  the  weight  of  the  roof  covering  and  from 
their  reciprocal  action  on  each  other.  These  strains  are 
transmitted  to  the  tie-beam,  causing  a  strain  of  tension  upon 
it.  The  struts  resist  the  cross  strain  upon  the  rafters  and 
prevent  them  from  sagging ;  and  the  king  and  queen  posts 
prevent  the  tie  and  straining  beams  from  sagging  and  give 
points  of  support  to  the  struts.  The  short  rafters  and  strain- 
ing beam  form  points  of  support  which  resist  the  cross  strain 
on  the  principal  rafters,  and  support  the  strain  on  the  queen 
posts. 

694.  Ties  and  Braces  for  Detached  Frames .  When  a 
series  of  frames  concur  to  one  end,  as,  for  example,  the  main 
beams  of  a  bridge,  the  trusses  of  a  roof,  ribs  of  a  centre,  etc., 
they  require  to  be  tied  together  and  stiffened  by  other  beams 
to  prevent  any  displacement  and  warping  of  the  frames. 
For  this  purpose  beams  are  placed  in  a  horizontal  position 
and  notched  upon  each  frame  at  suitable  points  to  connect 
the  whole  together ;  while  others  are  placed  crossing  each 
other,  in  a  diagonal  direction,  between  each  pair  of  frames, 
with  which  they  are  united  by  suitable  joints,  to  stiffen  the 
frames  and  prevent  them  from  yielding  to  any  lateral  effort. 
Both  the  ties  and  the  diagonal  braces  may  be  either  of  single 
beams,  or  of  beams  in  pairs,  so  arranged  as  to  embrace 
between  them  the  part  or  the  frames  with  which  they  are 
connected. 


396 


CIVIL  ENGINEERHSTG. 


695.  Iron  Roof  Trusses.  Frames  of  iron  for  roofs  have 
been  made  either  entirely  of  wrought  iron,  or  of  a  combina- 
tion of  wrought  and  cast  iron,  or  of  these  two  last  materials 
combined  with  timber.  The  combinations  for  the  trasses  of 
roofs  of  iron  are  in  all  respects  the  same  as  in  those  for  tim- 
ber trasses.  The  parts  of  the  truss  subjected  to  a  cross  strain, 
or  to  one  of  compression,  are  arranged  to  give  the  most  suit- 


Fig.  210 — ^Represents  the  half  of  a  trusi  for  the  same  building  composed  of  wronght  and 

cast  iron, 
a,  a,  feathered  stmts  of  cast-iron. 
6,  b,  suspension  bars  in  pairs. 
m,  n,  tie  and  straining  bars. 

e,  c,  and/,/,  cross  sections  of  beams  resting  in  the  cast-iron  sockets  connected  with  the  sus- 
pension bars. 

able  forms  for  strength,  and  to  adapt  them  to  the  object  in 
view.  The  parts  subjected  to  a  strain  of  extension,  as  the 
tie-beam  and  king  and  queen  posts,  are  made  either  of 
wrought  iron  or  timber,  as  may  be  found  best  adapted  to  the 
particular  end  proposed. 

The  joints  are  in  some  cases  arranged  by  inserting  the  ends 


BOOFS. 


397 


of  the  beams,  or  bars,  in  cast-iron  sockets,  or  shoes  of  a  suita- 
ble  form;  in  others  the  beams  are  united  by  joints  arranged* 
like  those  for  timber  frames,  the  joints  in  all  cases  being 
secured  by  wrought-iron  bolts  and  keys.    (Figs.  210.  211  and 
212.) 


Fig.  211 — Represents  the  half  of  a  truss  of  wrought  iron  for  the  new  Houses  of  Parliament, 
England.  The  pieces  of  this  truss  are  formed  of  bars  of  a  rectangular  section.  The  joints 
axe  secured  by  cast-iron  sockets,  within  which  the  ends  of  the  bars  are  secured  by  screw 
tx)lts. 


696.  Fig.  213  shows  a  very  common  form  of  the  roofs  of 
gas-houses. 

This  here  shown  is  supposed  to  be  made  entirely  of  iron 
At  the  ridge  is  a  ventilator  to  allow  the  escape  of  gases. 
The  manner  of  joining  the  parts  is  sufficiently  shown  in  the 


398 


CIVIL  ENGINEERING. 


Fig.  212— Represcntg  the  aiv 

rangements  of  the  parts  at 
the  joint  c  in  Fig.  210. 
A,  side  view  of  the  pieces 
and  joint. 

a,  princiijal  rafter  of  the 
cross  section  B. 

b,  common  rafter  of  the  cross 
section  C. 

c,  cross  section  of  purlins  and 
joint  for  fastening  the  com- 
mon rafters  to  the  purlins. 

d,  cast-iron  socket  arranged 
to  confine  the  pieces  a,  6, 
c,  e. 


Pig.  213.— Ordinary  roof  of  a  gas-house.  A,  B,  Is  the  main  rafter. 

a,  a'  a"  are  vertical  tie-roda. 

b,  b'  b"  are  braces, 
C,  D,  is  the  main  tie. 
E,  F,  is  the  ventilator, 

697.  Fig.  214  shows  a  mode  of  secondary  trussing.  A  is  a 
strut  for  supporting  the  middle  of  the  main  rafter.  The 
lower  end  of  A  is  secured  to  a  block  which  is  supported  by 
the  tie-rods  B  and  D.  The  tie-rods  C  and  D  serve  the  office 
of  a  single  tie  for  supporting  the  lower  end  of  E.    In  this 


EOOFS. 


399 


way  the  rod  D  performs  a  double  office.  It  may  be  question- 
able whether  this  arrangement  is  as  good  as  it  would  be  to 
have  one  continuous  rod  pass  from  E  to  F,  and  another 
rod  (D)  to  act  with  B. 


Fig.  214— A  is  a  strut,  the  lower  end  of  which  is  supported  by  the  ties  B  and  D.  C  and  D 
serve  the  office  of  a  continuous  tie  for  supporting  the  lower  end  of  the  strut  E. 


It  may  be  observed  that  in  this  Fig.  the  tie-rods  are  in- 
clined and  much  longer  than  the  struts,  which  is  the  reverse 
of  the  condition  in  Fig.  213.  If  iron  only  is  used  the  arrange- 
ment of  Fig.  214  will  generally  be  the  most  economical,' 
but  if  wooden  struts  are  used  the  plan  of  Fig.  213  may  be 
preferable. 


Fig.  215w 


698.  Depot  Roof  Truss,  Fig.  215  shows  a  truss  which 
has  been  used  in  many  cases  for  supporting  the  roofs  of  depots 
and  of  other  large  buildings.    The  passenger  depot  of  the 


400 


CrVTL  ENGINEERING. 


Michigan  Central  ^Railroad  at  Chicago  was  built  after  this 
plan.  It  was  destroyed  by  the  great  nre  in  1871.  The  plan 
of  the  arch  is  a  Howe  truss,  having  curved  wooden  chords, 
wooden  braces  and  iron  ties  to  connect  the  two  chords.  The 
truss  formed  an  arch,  the  thrust  of  which  was  resisted  by  a 
long  horizontal  tie-rod. 

The  same  style  was  adopted  in  the  new  roof  over  the  depot 
at  Troy,  New  York ;  and  the  Grand  Central  Depot  in  New 
York  City. 

699.  A  novel  plan  was  used  in  making  the  roof  over  the 
rolling-mills  at  Milwaukee,  Wis.  An  arch  was  made  of 
boards  so  placed  as  to  break  joints  and  form  a  rib  about  a 
foot  wide  and  eighteen  inches  deep,  and  one  hundred  and 
eighty  feet  span.  The  boards  were  bolted  together  so  as  to 
make  the  rib  continuous,  and  then  the  upper  part  of  the  arch 
w^as  trussed  after  the  Howe  plan.  The  main  objects  of  this 
plan  were  cheapness  and  to  secure  the  whole  inclosed  area 
free  from  posts  or  other  similar  obstructions.  But  it  was 
found  that  the  arch  was  too  weak,  especially  when  required 
to  carry  the  large  ventilator  which  was  placed  over  it,  and 
posts  were  afterwards  added. 

700.  Roofs  and  Domes.  In  some  cases — especially  in  state 
buildings — domes  are  placed  upon  roofs  for  architectural  effect. 

Fig.  216. 


Fig.  217. 

Figs.  216  and  217— Are  two  trusses,  which  are  made  in  pairs,  and  are  placed  fourteen  Indies 
apart,  for  supporting  part  ot  the  dome  (octagonal)  of  the  State  capitol  at  Montpelier,  Vt. 
a  a  a  are  the  short  timbers  for  connecting  the  two  trusses. 
A  is  a  timber  resting  upon  the  cross  pieces  a  a  a, 
C  is  a  post  of  the  dome  resting  upon  the  piece  A. 
Span,  sixty-seven  feet  four  inches. 


i 


EOOFS. 


401 


4:02 


CIVIL  ENGINEERING. 


The  dome  of  the  State  capitol,  Yermont,  rests  upon  wooden 
trusses  (Figs.  216  and  217),  having  a  span  of  sixtj-seven  feet 
four  inches.  The  trusses  are  supported  at  the  ends  only.  They 
are  placed  in  pairs,  fourteen  inches  apart.  The  Fig.  shows 
two  pairs.  They  are  connected  by  short  cross  beams,  a  a  y 
upon  which  rest  other  timbers.  A,  for  receiving  the  posts,  C, 
of  the  dome.  It  is  profitable  for  the  student  to  make  a  careful 
study  of  the  details  of  this  structure. 

Where  the  thrust  is  severe  especial  care  should  be  taken  to 
secure  a  good  bearing  for  the  ends  of  the  timbers.  The  lower 
ends  of  the  main  rafters  tend  to  shear  the  main  tie  at  its  ends, 
and  to  prevent  this  action  they  should  enter  the  tie  at  a 
reasonable  distance  from  its  ends.  The  bearing  pieces  are  of 
white  oak,  and  the  rest  of  the  timber  is  spruce.  The  trusse  s 
are  constructed  differently^,  because  the  posts  of  the  dome 
bear  upon  them  in  different  places. 

701.  Roof  over  the  large  hall  of  the  University  of 
Michigan.  This  truss  and  dome  presents  a  very  novel  fea- 
ture (Fig.  218),  inasmuch  as  a  part  of  the  dome  rests  directly, 
or  nearly  so,  upon  the  posts  which  support  the  roof,  while  the 
other  part  rests  directly  upon  the  trusses  which  support  the 
roof.  The  span  is  eighty  feet  in  the  clear,  and  the  depth  of  the 
trusses  is  sixteen  feet.  The  main  rafters  are  pieces  of  solid 
pine  fourteen  inches  wide  by  sixteen  inches  deep.  They  are 
not  of  equal  length,  the  longer  ones  having  a  horizontal  run 
of  forty-seven  feet,  and  the  shorter  ones  thirty-three  feet. 
The  secondary  trussing  is  distributed  according  to  the  strains. 
The  dome  is  thirty  feet  in  diameter  at  the  base. 

The  ceiling  of  the  large  hall  being  attached  directly  to 
trusses,  it  was  necessary  to  make  very  strong  trusses,  so  that 
the  action  of  the  wind  upon  the  dome,  and  also  the  effect  of 
the  changes  of  temperature  might  not  so  disturb  the  trusses 
by  causing  them  to  deflect,  as  to  destroy  the  ceiling.  (For  a 
computation  of  the  parts,  see  Wood's  Bridges  and  lioo/Sj  pp. 
194-211. 


EOADS. 


403 


CHAPTEE  YII. 

KOADS. 

I.  COMMON  EOADS.     H.  KAILEOADS. 

702.  In  establishing  a  line  of  internal  communication  of 
any  character,  whether  it  be  an  ordinary  road,  railroad,  or 
canal,  the  main  considerations  to  which  the  attention  of  the 
engineer  must  be  directed  in  the  outset  are:  1,  the  probable 
character  and  amount  of  traffic  over  the  line  ;  2,  the  wants  of 
the  community  in  the  neighborhood  of  the  line ;  3,  the  nat- 
ural features  of  the  country,  between  the  points  of  arrival 
and  de/parture^  as  regards  their  adaptation  to  the  proposed 
communication. 

As  the  last  point  alone  comes  exclusively  within  the  prov- 
ince of  the  engineer's  art,  and  within  the  limits  prescribed  to 
this  work,  attention  will  be  confined  solely  to  its  consideration, 

703.  Reconnaissance.  A  thorough  examination  and  study 
of  the  ground  by  the  eye,  termed  a  reGonnaissance^  is  an  in- 
dispensable preliminary  to  any  more  accurate  and  minute 
survey  by  instruments,  to  avoid  loss  of  time,  as  by  this  more 
rapid  operation  any  ground  unsuitable  for  the  proposed  line 
will  be  as  certainly  detected  by  a  person  of  some  experience, 
as  it  could  be  by  the  slow  process  of  an  instrumental  survey. 
Before,  however,  proceeding  to  make  a  reconnaissance,  a  care- 
ful inspection  of  the  general  maps  of  that  portion  of  the 
country  through  which  the  communication  is  to  pass  will 
facilitate,  and  may  considerably  abridge  the  labors  of  the  en- 
gineer ;  as  from  the  natural  features  laid  down  upon  them, 

Particularly  the  direction  of  the  water-courses,  he  will  at  once 
e  able  to  detect  those  points  which  will  be  favorable,  or 
otherwise,  to  the  general  direction  selected  for  the  line.  This 
will  be  sufficiently  evident  when  it  is  considered — 1,  that  the 
water-courses  are  necessarily  the  lowest  lines  of  the  valleys 
through  which  they  flow,  and  that  their  direction  must  also  be 
that  of  the  lines  of  greatest  declivity  of  their  respective  val- 
leys ;  2,  that  from  the  position  of  the  water-courses  the  posi- 
tion also  of  the  high  grounds  by  which  they  are  separated 
naturally  follows,  as  well  as  the  approximate  position  at  least 


404 


CIVIL  ENGINEEEING. 


of  the  ridges,  or  highest  lines  of  the  high  grounds,  which 
separate  their  opposite  slopes,  and  which  are  at  the  same  time 
the  lines  of  greatest  declivity  common  to  these  slopes,  as  the 
water-courses  are  the  corresponding  lines  of  the  slopes  that 
form  the  valleys. 

Keeping  these  facts  (which  are  susceptible  of  rigid  mathe- 
matical demonstration)  in  view,  it  will  be  practicable,  from  a 
careful  examination  of  an  ordinary  general  map,  if  accurately 
constructed,  not  only  to  trace,  with  considerable  accuracy,  the 
general  direction  of  the  ridges  from  having  that  of  the  water- 
courses, but  also  to  detect  those  depressions  in  them  which 
will  be  favorable  to  the  passage  of  a  communication  intended 
to  connect  two  main  or  two  secondary  valleys.  The  follow- 
ing illustrations  may  serve  to  place  this  subject  in  a  clearer 
aspect. 

If,  for  example,  it  be  found  that  on  any  portion  of  a  map 
the  water-courses  seem  to  diverge  from  or  converge  towards 
one  point,  it  will  be  evident  that  the  ground  in  the  first  case 
must  be  the  common  source  or  supply  of  the  water-courses, 
and  therefore  the  highest ;  and  in  the  second  case  that  it  is 
the  lowest,  and  forms  their  common  recipient. 

If  two  water-courses  flow  in  opposite  directions  from  a  com- 
mon point,  it  will  show  that  this  is  the  point  from  which  they 
derive  their  common  supply,  at  the  head  of  their  respective 
valleys,  and  that  it  must  be  fed  by  the  slopes  of  high  grounds 
above  this  point ;  or,  in  other  words,  that  the  valleys  of  the 
two  water-courses  are  separated  by  a  chain  of  high  grounds, 
which,  at  the  point  where  it  crosses  them,  presents  a  depres- 
sion in  its  ridge,  which  would  be. the  natural  position  for  a 
communication  connecting  the  two  valleys. 

If  two  water-courses  flow  in  the  same  direction  and  parallel 
to  each  other,  it  will  simply  indicate  a  general  inclination  of 
the  ridge  between  them,  in  the  same  direction  as  that  of  the 
water-courses.  The  ridge,  however,  may  present  in  its  course 
elevations  and  depressions,  which  will  be  indicated  by  the 
points  in  which  the  water-courses  of  the  secondary  valleys, 
on  each  side  of  it,  intersect  each  other  on  it ;  and  these  will 
be  the  lowest  points  at  w^hich  lines  of  communication,  through 
the  secondary  valleys,  connecting  the  main  water-courses, 
would  cross  the  dividing  ridge. 

If  two  water-courses  flow  in  the  same  direction,  and  paral- 
lel to  each  other,  and  then  at  a  certain  point  assume  divergent 
directions,  it  will  indicate  that  this  is  the  lowest  point  of  the 
ridge  between  them. 

If  two  water-courses  flow  in  parallel  but  opposite  directions, 


COMMON  EOADS. 


405 


(leprepsions  in  the  ridge  between  them  will  be  shown  by 
the  meeting  of  the  water-courses  of  the  secondary  valleys  on 
the  ridge  ;  or  by  an  approach  towards  each  other,  at  any  point, 
of  the  two  principal  water-courses. 

Furnished  with  the  data  obtained  from  the  maps,  the  char- 
acter of  the  ground  should  be  carefully  studied  both  ways 
by  the  engineer,  first  from  the  point  of  departure  to  that  oi 
arrival,  and  then  returning  from  the  latter  to  the  former,  as 
without  this  double  traverse  natural  features  of  essential  im- 
portance might  escape  the  eye. 

704.  Surveys.  From  the  results  of  the  reconnaissance, 
the  engineer  will  be  able  to  direct  understandingly  the  requi- 
site surveys,  which  consist  in  measuring  the  lengths,  determin- 
ing the  directions,  and  ascertaining  both  the  longitudinal  and 
cross  levels  of  the  different  routes,  or,  as  they  are  termed, 
trial-lines ,  with  sufficient  accuracy  to  enable  him  to  make  a 
comparative  estimate  both  of  their  practicability  and  cost. 
As  the  expense  of  making  the  requisite  surveys  is  usually  but 
a  small  item  compared  with  that  of  constructing  the  commu- 
nication, no  labor  should  be  spared  in  running  every  practica- 
ble line,  as  otherwise  natural  features  might  be  overlooked 
which  might  have  an  important  influence  on  the  cost  of  con- 
struction. 

705.  Map  and  Memoir.  The  results  of  the  surveys  are 
accurately  embodied  in  a  map  exhibiting  minutely  the  topo- 
graphical features  and  sections  of  the  different  trial-lines, 
and  in  a  memoir  which  should  contain  a  particular  descrip- 
tion of  those  features  of  the  ground  that  cannot  be  shown  on 
a  map,  with  all  such  information  on  other  points  that  may 
be  regarded  as  favorable,  or  otherwise,  to  the  proposed  com- 
munication ;  as,  for  example,  the  nature  of  the  soil,  that  of 
the  water-courses  met  with,  etc.,  etc. 

706.  Location  of  Common  Roads.  In  selecting  among 
the  different  trial-lines  of  the  survey  the  one  most  suitable  to 
a  common  road,  the  engineer  is  less  restricted,  from  the 
nature  of  the  conveyance  used,  than  in  any  other  kind  of 
communication.  The  main  points  to  which  his  attention 
should  be  confined  are :  1,  to  connect  the  points  of  arrival 
and  departure  by  the  most  direct,  or  shortest  line  ;  2,  to 
avoid  unnecessary  ascents  and  descents,  or,  in  other  words,  to 
reduce  the  ascents  and  descents  to  the  smallest  practicable 
limit ;  3,  to  adopt  such  suitable  slopes,  or  gradients,  for  the 
axis,  or  centre  line  of  the  road,  as  the  nature  of  the  convey- 
ance may  demand ;  4,  to  give  the  axis  such  a  position  with 
regard  to  the  surface  of  the  ground  and  the  natural  obstacles 


m 


CrVTL  ENGINEEHmG. 


to  be  overcome,  that  the  cost  of  construction  for  the  excava- 
tions and  embankments  required  by  the  gradients,  and  for 
the  bridges  and  other  accessories,  shall  be  reduced  to  the 
lowest  amount. 

707.  Deviations  from  the  right  line  drawn  on  the  map,  be- 
tween the  points  of  arrival  and  departure,  will  be  often  de- 
manded by  the  natural  features  of  the  ground.  In  passing 
the  dividing  ridges  of  main,  or  secondary  valleys,  for  ex- 
ample, it  will  frequently  be  found  more  advantageous,  both 
for  the  most  suitable  gradients,  and  to  diminish  the  amount 
of  excavation  and  embankment,  to  cross  the  ridge  at  a  lower 
point  than  the  one  in  which  it  is  intersected  by  the  right  line, 
deviating  from  the  right  line  either  towards  the  heady  or 
Tipper  part  of  the  valley,  or  towards  its  outlet,  according  to 
the  advantages  presented  by  the  natural  features  of  the 
ground,  both  for  reducing  the  gradients  and  the  amount  of 
excavation  and  embankment. 

Where  the  right  line  intersects  either  a  marsh  or  water- 
course, it  may  be  found  less  expensive  to  change  the  direction, 
avoiding  the  marsh,  or  intersecting  the  water-course  at  a 
point  w^iere  the  cost  of  construction  of  a  bridge,  or  of  the 
approaches  to  it,  will  be  more  favorable  than  the  one  in 
which  it  is  intersected  by  the  right  line. 

Clianges  from  the  direction  of  the  right  line  may  also  be 
favorable  for  the  purpose  of  avoiding  the  intersection  of 
secondary  w^ater-courses ;  of  gaining  a  better  soil  for  the 
roadway  ;  of  giving  a  better  exposure  of  its  surface  to  the 
sun  and  wind  ;  or  of  procuring  better  materials  for  the  road- 
covering. 

By  a  careful  comparison  of  the  advantages  presented  by 
these  different  features,  the  engineer  will  be  enabled  to 
decide  how  far  the  general  direction  of  the  right  line  may  be 
departed  from  with  advantage  to  the  location.  By  choosing 
a  more  sinuous  course  the  length  of  the  line  will  often  not 
be  increased  to  any  very  considerable  degree,  while  the  cost 
of  construction  may  be  greatly  reduced,  either  in  obtaining 
more  favoral)le  gi*aclients,  or  in  lessening  the  amount  of  ex- 
cavation and  embankment. 

708.  When  the  points  of  arrival  and  departure  are  upon 
different  levels,  as  is  usually  the  case,  it  will  seldom  be  prac- 
ticable to  connect  them  by  a  continual  ascent.  Tiie  most 
that  can  be  done  will  be  to  cross  tht  dividing  ridges  at  their 
lowest  points,  and  to  avoid,  as  far  as  practicable,  the  intersec- 
tion of  considerable  secondary  valleys  which  might  require 
any  considerable  ascent  on  one  side  and  descent  on  the  other. 


COMMON  ROADS. 


407 


709.  The  gradients  upon  common  roads  will  depend  npon 
the  kind  of  material  used  for  the  road-covering,  and  upon  the 
state  in  which  the  road-siirface  is  kept.  The  gradient  in  all 
cases  should  be  less  than  the  angle  of  repose^  or  of  that  in- 
clination of  the  axis  of  the  road  in  which  the  ordinary 
vehicles  for  transportation  would  remain  at  a  state  of  rest,  or, 
if  placed  in  motion,  would  descend  by  the  action  of  gravity 
with  uniform  velocity. 

The  gradients  corresponding  to  the  angle  of  repose  have 
been  ascertained  by  experiments  made  upon  the  various  road- 
coverings  in  ordinary  use,  by  allowing  a  vehicle  to  descend 
alono:  a  road  of  variable  inclination  until  it  was  brou2:ht  to  a 
state  of  rest  by  the  retarding  force  of  friction;  also,  by  as- 
certaining the  amount  of  force,  termed  the  force  of  traction^ 
requisite  to  put  in  motion  a  vehicle  with  a  given  load  on  a 
level  road. 

The  following  are  the  results  of  experiments  made  by  Mr. 
Macneill,  in  England,  to  determine  the  force  of  traction  for 
one  ton  upon  level  roads  : — 
No.  1.  Good  pavement,  the  force  of  traction  is. .  . .     33  lbs. 

"   2.  Broken-stone  surface  laid  on  an  old  flint  road     65  " 


"   3.  Gravel  road   147  " 

"   4.  Broken-stone  surface  on  a  rough  pavement 

bottom   46  " 

"   .5.  Broken-stone  surface  on  a  bottom  of  beton . .  46  " 


From  this  it  appears  that  the  angle  of  repose  in  the  first 
case  is  repiesented  by  ^ff  05  -eV  nearly  ;  and  that  the  slope 
of  the  road  should  therefore  not  be  greater  than  one  perpendic- 
ular to  sixty-eight  in  length  ;  or  that  the  height  to  be  overcome 
must  not  be  greater  than  one  sixty-eighth  of  the  distance  be- 
tween the  two  points  measured  along  the  road,  in  order  that 
the  force  of  friction  may  counteract  that  of  gravity  in  the 
direction  of  the  road. 

A  similar  calculation  will  show  that  the  angle  of  repose  in 
the  other  cases  will  be  as  follows  : 

No.  2  1  to  35  nearly. 

"  3  1  to  15 

"  4  and  5  1  to  49  " 

These  numbers,  which  give  the  angle  of  repose  between  -^-^ 
and  ^  for  the  kinds  of  road-covering  Nos.  2  and  4  in  most 
ordinary  use,  and  corresponding  to  a  road-surface  in  good 
order,  may  be  somewhat  increased,  to  from  to  -g^j,  for  the 
ordinary  state  of  the  surface  of  a  well-kept  road,  without 
there  being  any  necessity  for  applying  a  brake  to  the  wheels 
in  descending,  or  going  out  of  a  trot  in  ascending.  The 


408 


OmL  ENGINEERING. 


steepest  gradient  that  can  be  allowed  on  roads  with  a  broken- 
stone  covering  is  about  -^^^  as  this,  from  experience,  is  found 
to  be  about  the  angle  of  repose  upon  roads  of  this  character 
in  the  state  in  which  they  are  usually  kept.  Upon  a  road 
with  this  inclination,  a  horse  can  draw  at  a  walk  his  usual 
load  for  a  level  without  requiring  the  assistance  of  an  extra 
horse ;  and  experience  has  fai'ther  shown  that  a  horse  at  the 
usual  walking  pace  will  attain,  with  less  apparent  fatigue,  the 
summit  of  a  gradient  of  in  nearly  the  same  time  that  he 
would  require  to  reach  the  same  point  on  a  trot  over  a  gra- 
dient of  -J3-. 

A  road  on  a  dead  level,  or  one  with  a  continued  and  uni- 
form ascent  between  the  points  of  arrival  and  depai'ture,  where 
they  lie  upon  different  levels,  is  not  the  most  favorable  to  the 
draft  of  the  horse.  Each  of  these  seems  to  fatigue  him  more 
than  a  line  of  alternate  ascents  and  descents  of  slight  gra- 
dients ;  as,  for  example,  gradients  of  upon  which  a  horse 
will  draw  as  heavy  a  load  with  the  same  speed  as  upon  a  hori- 
zontal road. 

The  gradients  should  in  all  cases  be  reduced  as  far  as  prac- 
ticable, as  the  extra  exertion  that  a  horse  must  put  forth  in 
overcoming  heavy  gradients  is  very  considerable  ;  they  should 
as  a  general  rule,  therefore,  be  kept  as  low  at  least  as 
wherever  the  ground  will  admit  of  it.  This  can  generally  be 
effected,  even  in  ascending  steep  hill-sides,  by  giving  the  axis 
of  the  road  a  zigzag  direction,  connecting  the  straight  por- 
tions of  the  zigzags  by  circular  arcs.  The  gradients  of  the 
curved  portions  of  the  zigzags  should  be  reduced,  and  the 
roadway  also  at  these  points  be  widened,  for  the  safety  of  ve- 
hicles descending  rapidly.  The  width  of  the  roadway  may 
be  increased  about  one-fourth,  when  the  angle  between  the 
straight  portions  of  the  zigzags  is  from  120°  to  90°  ;  and  the 
increase  should  be  nearly  one-half  where  the  angle  is  from 
90°  to  60°. 

710.  Having  laid  down  upon  the  map  the  approximate  loca- 
tion of  the  axis  of  the  road,  a  comparison  can  then  he  made 
hetvjeen  the  solid  contents  of  the  excavations  and  embank- 
ments, which  should  be  so  adjusted  that  they  shall  balance 
each  other,  or,  in  other  words,  the  necessary  excavations  shall 
furnish  sufficient  earth  to  form  the  embankments.  To  effect 
this,  it  will  frequently  be  necessary  to  alter  the  first  location, 
by  shifting  the  position  of  the  axis  to  the  right  or  left  of  the 
position  first  assumed,  and  also  by  changing  the  gradients 
within  the  prescribed  limits.  This  is  a  problem  of  very  con- 
siderable intricacy,  whose  solution  can  only  be  arrived  at  by 


COMMON  ROADS. 


409 


successive  approximations.  For  this  purpose,  the  line  must 
be  subdivided  into  several  portions,  in  each  of  which  the 
equalization  should  be  attempted  independently  of  the  rest, 
instead  of  trying  a  general  equalization  for  the  whole  line  at 
once. 

In  the  calculations  of  solid  contents  required  in  balancing 
the  excavations  and  embankments,  the  most  accurate  method 
consists  in  subdividing  the  different  solids  into  others  of  the 
most  simple  geometrical  forms,  as  prisms,  prismoids,  wedges, 
and  pyramids,  whose  solidities  are  readily  determined  by  the 
ordinary  rules  for  the  mensuration  of  solids.  As  this  pro- 
cess, however,  is  frequently  long  and  tedious,  other  methods 
requiring  less  time,  but  not  so  accurate,  are  generally  pre- 
ferred, as  their  results  give  an  approximation  sufficiently 
near  the  true  for  most  practical  purposes.  They  consist  in 
taking  a  number  of  equidistant  profiles,  and  calculating  the 
solid  contents  between  each  pair,  either  by  multiplying  the 
half  sum  of  their  areas  by  the  distance  between  them,  or  else 
by  taking  the  profile  at  the  middle  point  between  each  pair, 
and  multiplying  its  area  by  the  same  length  as  before.  The 
latter  method  is  the  more  expeditious  ;  it  gives  less  than  the 
true  solid  contents,  but  a  nearer  approximation  than  the  for- 
mer, which  gives  more  than  the  true  solid  contents,  whatever 
may  be  the  form  of  the  ground  between  each  pair  of  cross 
profiles. 

In  calculating  the  solid  contents,  allowance  must  be  made 
for  the  difference  in  bulk  between  the  different  kinds  of  earth 
when  occupying  their  natural  bed  and  when  made  into  em- 
bankment. From  some  careful  experiments  on  this  point 
made  by  Mr.  Elwood  Morris,  a  civil  engineer,  and  published 
in  the  Journal  of  the  Franklin  Institute^  it  appears  that  light 
sandy  earth  occupies  the  same  space  both  in  excavation  and 
embankment ;  clayey  earth  about  one-tenth  less  in  embankment 
than  in  its  natural  bed  ;  gravelly  earth  also  about  one-twelfth 
less;  rock  in  large  fragments  about ' five-twelfths  more,  and 
in  small  fragments  about  six-tenths  more. 

711.  Another  problem  connected  with  the  one  in  question 
is  that  of  determining  the  lead^  or  the  mean  distance  to  which 
the  earth  taken  fro77i  the  excavations  must  he  carried  to  form 
the  ernbanhments.  From  the  manner  in  which  the  earth  is 
usually  transported  from  the  one  to  the  other,  this  distance  is 
usually  that  between  the  centre  of  gravity  of  the  solid  of  ex- 
cavation and  that  of  its  corresponding  embankment.  What- 
ever disposition  may  be  made  of  the  solids  of  excavation,  it 
is  important,  so  far  as  the  cost  of  their  removal  is  concerned, 


410 


CIVIL  ENGINEEEING. 


that  the  lead  should  be  the  least  possible.  The  solation  of 
the  problem  under  this  point  of  view  will  frequently  be  ex- 
tremely intricate,  and  demand  the  application  of  all  tlie  re- 
sources of  the  higher  analysis.  One  general  principle, 
however,  is  to  be  ol)sei  ved  in  all  cases,  in  order  to  obtain  an 
approximate  solution,  which  is,  that  in  the  remo\  al  of  the 
✓  dinerent  portions  of  the  solid  of  excavation  to  their  corre- 
sponding positions  on  that  of  the  embankment,  the  paths 
passed  over  by  their  respective  centres  of  gravity  shall  not 
cross  each  other  either  in  a  horizontal  or  vertical  direction. 
This  may  in  most  cases  be  effected  by  intersecting  the  solids 
of  excavation  and  embankment  by  vertical  planes  in  the 
direction  of  the  removal,  and  by  removing  the  partial 
solids  between  the  planes  within  the  boundaries  marked  out 
by  them. 

712.  The  definitive  location  having  been  settled  by  again 
going  over  the  line,  and  comparing  the  features  of  the  ground 
with  the  results  furnished  by  the  preceding  operations,  gene- 
ral and  detailed  maps  of  the  different  divisions  of  the  defini- 
tive location  are  prepared,  which  should  give,  with  the 
utmost  accuracy,  the  longitudinal  and  cross  sections  of  the 
natural  ground,  and  of  the  excavations  and  embankments, 
with  the  horizontal  and  vertical  measurements  carefully  writ- 
ten upon  them,  so  that  the  superintending  engineer  may  have 
no  difficulty  in  setting  out  the  work  from  them  on  the 
ground. 

In  addition  to  these  maps,  which  are  mainly  intended  to 
guide,  the  engineer  in  regulating  the  earth-work,  detailed 
drawings  of  the  road-covering,  of  the  masonry  and  carpentry 
of  thie  bridges,  culverts,  etc.,  accompanied  by  written  specifi- 
cations of  the  manner  in  which  the  various  kind  of  work  is 
to  be  performed,  should  be  prepared  for  the  guidance  both 
of  the  engineer  and  workmen. 

713.  With  the  data  furnished  by  the  maps  and  drawings, 
the  engineer  can  proceed  to  set  out  the  line  on  the  ground. 
The  axis  of  the  road  is  determined  by  placing  stout  stakes  or 
pickets  at  equal  intervals  apart,  which  are  numbered  to  corre- 
spond with  the  same  points  on  the  map.  The  width  of  the 
roadway  and  the  lines  on  the  ground  corresponding  to  the 
side  slopes  of  the  excavations  and  embankments  are  laid  out 
in  the  same  manner,  by  stakes  placed  along  the  lines  of  the 
cross  profiles. 

Besides  the  numbers  marked  on  the  stakes,  to  indicate  their 
position  on  the  map,  other  numbers,  showing  the  depth  of  the 
excavations,  or  the  height  of  the  embankments  from  the  sur- 


COMMON  EOADS. 


411 


face  of  the  ground,  accompanied  by  the  letters  Cut.  Fill,  to 
indicate  a  cutting^  or  a  filling.,  as  the  case  may  be,  are  also 
added  to  guide  the  workmen  in  their  operations.  The  posi- 
tions of  the  stakes  on  the  ground,  which  show  the  principal 
points  of  the  axis  of  the  road,  should,  moreover,  be  laid  down 
on  the  map  with  great  accuracy,  by  ascertaining  their  bear- 
ing and  distances  from  any  fixed  and  marked  objects  in  their 
vicinity,  in  order  that  the  points  may  be  readily  found  should 
the  stakes  be  subsequently  misplaced. 

714.  Earth-Work.  This  term  is  applied  to  whatever  re- 
lates to  the  construction  of  the  excavations  and  embankments, 
to  prepare  them  for  receiving  the  road-covering. 

715.  In  forming  the  excavations,  the  inclination  of  the  side 
slopes  demands  peculiar  attention.  This  inclination  will  de- 
pend on  the  nature  of  the  soil,  and  the  action  of  the  atmos- 
phere and  internal  moisture  upon  it.  In  common  soils,  as 
ordinary  garden  earth  formed  of  a  mixture  of  clay  and  sand, 
compact  clay,  and  compact  stony  soils,  although  the  side 
slopes  would  withstand  very  well  the  effects  of  the  weather 
with  a  greater  inclination,  it  is  best  to  give  them  two  base  to 
one  perpendicular,  as  the  surface  of  the  roadway  will,  by  this 
arrangement,  be  w^ell  exposed  to  the  action  of  the  sun  and 
air,  which  will  cause  a  rapid  evaporation  of  the  moisture  on 
the  surface.  Pure  sand  and  gravel  may  require  a  greater 
slope,  according  to  circumstances.  In  all  cases  where  the 
depth  of  the  excavation  is  great,  the  base  of  the  slope  should 
be  increased.  It  is  not  usual  to  use  any  artificial  means  to 
protect  the  surface  of  tlie  side  slopes  from  the  action  of  the 
weather ;  but  it  is  a  precaution  which,  in  the  end,  will  save 
much  labor  and  expense  in  keeping  the  roadway  in  good  or- 
der. The  simplest  means  which  can  be  used  for  this  purpose 
consist  in  covering  the  slopes  with  good  sods  (Fig.  219),  or 

Fig.  219.  Cross-section  of  a  road  in 
excavation. 

A,  road-surface. 

B,  side  slopes. 

C,  top  surface-drain. 

else  with  a  layer  of  vegetable  mould  about  four  inches  thick, 
carefully  laid  and  sown  with  grass-seed.  These  means  will 
be  amply  sufficient  to  protect  the  side  slopes  from  injury 
when  they  are  not  exposed  to  any  other  causes  of  deteriora- 
tion than  the  wash  of  the  rain,  and  the  action  of  frost  on  the 
ordinary  moisture  retained  by  the  soil. 

The  side  slopes  form  usually  an  unbroken  surface  from  the 


412 


CIVIL  ENGINEERING. 


foot  to  the  top.  But  in  deep  excavations,  and  particnlarly  in 
soils  liable  to  thev  arc  sometimes  formed  with  horizon- 

tal offsets,  termed  benches,  which  are  made  a  few  feet  wide, 
and  have  a  ditch  on  the  inner  side  to  receive  the  surface 
water  from  the  portion  of  the  side  slope  above  them.  These 
benches  catch  and  retain  the  earth  that  may  fall  from  the 
portion  of  the  side  slope  above. 

When  the  side  slopes  are  not  protected,  it  will  be  well,  in 
localities  where  stone  is  plenty,  to  raise  a  small  wall  of  dry 
stone  at  the  foot  of  the  slopes,  to  prevent  the  wash  of  the 
slopes  from  being  carried  into  the  roadway. 

A  covering  of  brushwood,  or  a  thatch  of  straw,  may  also  be 
used  with  go jd  effect ;  but,  from  their  perishable  nature,  they 
will  require  frequent  renewal  and  repairs. 

In  excavations  through  solid  rock,  which  does  not  disinte- 
grate on  exposure  to  the  atmosj^here,  the  side  slopes  might  be 
made  perpendicular ;  but  as  this  would  exclude,  in  a  great 
degree,  the  action  of  the  sun  and  air,  w^hich  is  essential  to 
keeping  the  road-surface  dry  and  in  good  order,  it  will  be 
necessary  to  make  the  side  slopes  with  an  inclination,  varying 
from  one  base  to  one  perpendicular,  to  one  base  to  two  per- 
pendicular, or  even  greater,  according  to  the  locality ;  the  in- 
clination of  the  slope  on  the  south  side  in  northern  latitudes 
being  greatest,  to  expose  better  the  road-surface  to  the  sun's 
rays. 

The  slaty  rocks  generally  decompose  rapidly  on  the  sur- 
face, when  CAjjosed  to  moisture  and  the  action  of  frost.  The 
side  slopes  in  rocks  of  this  character  may  be  cut  into  steps 


Fig.  220, 


(Fig.  220),  and  then  be  covered  by  a  layer  of  vegetable 
mould  sown  in  grass-seed,  or  else  the  earth  may  be  sodded  in 
the  usual  way. 

716.  The  stratified  soils  and  rocks,  in  which  the  strata  have 
a  dip,  or  inclination  to  the  horizon,  are  liable  to  slips,  or  to 
give  way  by  one  stratum  l)ecoming  detached  and  sliding  on 
another,  which  is  caused  either  from  the  action  of  frost,  or 
from  the  pressure  of  water,  which  insinuates  itself  between 


COIVCMON  EOADS. 


413 


the  strata.  The  worst  soils  of  this  character  are  those  formed 
of  alternate  strata  of  clay  and  sand ;  particularly  if  the  clay 
is  of  a  nature  to  become  semi-fluid  when  mixed  with  water. 
The  best  preventives  that  can  be  resorted  to  in  these  cases 
are  to  adopt  a  thorough  system  of  drainage,  to  prevent  the 
surface-water  of  the  ground  from  running  down  the  side 
slopes,  and  to  cut  off  all  springs  which  run  towards  the  road- 
way from  the  side  slopes.  The  surface-water  may  be  cut  off 
by  means  of  a  single  ditch  (Fig.  219)  made  on  the  up-hill  side 
of  the  road,  to  catch  the  water  before  it  reaches  the  slope  of 
the  excavation,  and  convey  it  off  to  the  natural  water-courses 
most  convenient ;  as,  in  almost  every  case,  it  will  be  found 
that  the  side  slope  on  the  down-hill  side  is,  comparatively 
speaking,  but  slightly  affected  by  the  surface-water. 

Where  slips  occur  from  the  action  of  springs,  it  frequently 
becomes  a  very  difficult  task  to  secure  the  side#slopes.  If  the 
sources  can  be  easily  reached  by  excavating  into  the  side 
slopes,  drains  formed  of  layers  of  fascines  or  brush-wood  may 
be  placed  to  give  an  outlet  to  the  water,  and  prevent  its  action 
upon  the  side  slopes.  The  fascines  may  be  covered  on  top 
with  good  sods  laid  with  the  grass  side  beneath,  and  the  exca- 
vation made  to  place  the  drain  be  filled  in  with  good  earth  well 
rammed.  Drains  formed  of  broken  stone,  covered  in  like 
manner  on  top  with  a  layer  of  sod  to  prevent  the  drain  from 
becoming  choked  wdth  earth,  may  be  used  under  the  same 
circumstances  as  fascine  drains.  Where  the  sources  are  not 
isolated,  and  the  whole  mass  of  the  soil  forming  the  side 
slopes  appears  saturated,  the  drainage  may  be  effected  by 
excavating  trenches  a  few  feet  wide  at  intervals  to  the  deptli 
of  some  feet  into  the  side  slopes,  and  filling  them  with  broken 
stone,  or  else  a  general  drain  of  broken  stone  may  be  made 
throughout  the  whole  extent  of  the  side  slope  by  excavating 
into  it.  When  this  is  deemed  necessary,  it  will  be  well  to 
arrange  the  drain  like  an  inclined  retaining-wall,  with  but- 
tresses at  intervals  projecting  into  the  earth  farther  than  the 
general  mass  of  the  drain.  The  front  face  of  the  drain 
should,  in  this  case,  also  be  covered  with  a  layer  of  sods  with 
the  grass  side  beneath,  and  upon  this  a  layer  of  good  earth 
should  be  compactly  laid  to  form  the  face  of  the  side  slopes. 
The  drahi  need  only  be  carried  high  enough  above  the  foot 
of  the  side  slope  to  tap  all  the  sources ;  and  it  should  be  sunk 
sufficiently  below  the  roadway-surface  to  give  it  a  secure 
footing. 

The  drainage  has  been  effected,  in  some  cases,  by  sinking 
wells  or  shafts  at  some  distance  behind  the  side  slopes,  from 


414 


CrVIL  ENGINEEKING. 


the  top  surface  to  the  level  of  the  bottom  of  the  excavation, 
and  leading  the  water  which  collects  in  them  by  pipes  into 
drains  at  the  foot  of  the  side  slopes.  In  others  a  narrow 
trench  has  been  excavated,  parallel  to  the  axis  of  the  road, 
from  the  top  surface  to  a  sufficient  depth  to  tap  all  the  sources 
which  flow  towards  the  side  slope,  and  a  drain  formed  either 
bj  filling  the  trench  wholly  with  broken  stone,  or  else  by  ar- 
ranging an  open  conduit  at  the  bottom  to  receive  the  water 
collected,  over  which  a  layer  of  brushwood  is  laid,  the  re- 
mainder of  the  trench  being  filled  with  broken  stone. 
717.  In  forming  the  embankments  (Fig.  221),  the  side 


Fig.  221. 

slopes  should  be  made  with  a  less  inclination  than  that  which 
the  earth  naturally  assumes ;  for  the  purpose  of  giving  them 
greater  durability,  and  to  prevent  the  width  of  the  top  sur- 
face, along  which  the  roadway  is  made,  from  diminishing  by 
every  change  in  the  side  slopes,  as  it  would  were  they  made 
with  the  natural  slope.  To  protect  the  side  slopes  more  ef- 
fectually, they  should  be  sodded,  or  sown  in  grass-seed  ;  and 
the  surface-water  of  the  top  should  not  be  allowed  to  run 
down  them,  as  it  would  soon  wash  them  into  gullies,  and  de- 
stroy the  embankment.  In  localities  where  stone  is  plenty,  a 
sustaining  wall  of  dry  stone  may  be  advantageously  substi- 
tuted for  the  side  slopes. 

To  prevent,  as  far  as  possible,  the  settling  which  takes 
place  in  embankments,  they  should  be  formed  with  great 
care ;  the  earth  being  laid  in  successive  layers  of  about  four 
feet  in  thickness,  and  each  layer  well  settled  with  rammers. 
As  this  method  is  very  expensive,  it  is  seldom  resorted  to  ex- 
cept in  works  which  require  great  care,  and  are  of  trifling  ex- 
tent. For  extensive  works,  the  method  usually  followed,  on 
account  of  economy,  is  to  embank  out  from  one  end,  carrying 
forward  the  work  on  a  level  with  the  top  surface.  In  this 
case,  as  there  must  be  a  want  of  compactness  in  the  mass,  it 
would  be  best  to  form  the  outsides  of  the  embankment  fii-st, 
and  to  gradually  fill  in  towards  the  centre,  in  order  that  the 
earth  may  arrange  itself  in  layers  with  a  dip  from  the  sides 
inwards:  this  will  in  a  e^reat  measure  counteract  any  ten- 
dency to  slips  outward.    The  foot  of  the  slopes  should  be  se- 


COMMON  KOADS. 


415 


cured  by  buttressing  them  either  by  a  low  stone  wall,  or  by 
formino^  a  slight  excavation  for  the  same  purpose. 

718.  When  the  axis  of  the  roadway  is  laid  out  on  the  side 
slope  of  a  hill,  and  the  road-surface  is  formed  partly  by  exca- 
vating and  partly  by  embanking  out,  the  usual  and  most 
simple  method  is  to  extend  out  the  embankment  gradually 
along  the  whole  line  of  excavation.  This  method  is  insecure, 
and  no  pains  therefore  should  be  spared  to  give  the  embank- 
ment a  good  footing  on  the  natural  surface  upon  which  it 
rests,  particularly  at  the  foot  of  the  slope.  For  this  purpose 
the  natural  surface  (Fig.  222)  should  be  cut  into  steps,  or  off- 


Fig.  222. 


sets,  and  the  foot  of  the  slope  be  secured  by  buttressing  it 
against  a  low  stone  wall,  or  a  small  terrace  of  carefully  ram- 
med earth. 

In  side-formings  along  a  natural  surface  of  great  inclina- 
tion, the  method  of  construction  just  explained  will  not  be 
sufficiently  secure ;  sustaining-walls  must  be  substituted  for 
the  side  slopes,  both  of  the  excavations  and  embankments. 
These  walls  may  be  made  simply  of  dry  stone,  when  the  stone 
can  be  procured  in  blocks  of  sufficient  size  to  render  this  kind 
of  construction  of  sufficient  stability  to  resist  the  pressure  of 
the  earth.  But  when  the  blocks  of  stone  do  not  offer  this 
security,  they  must  be  laid  in  mortar  (Fig.  223),  and  hydrau- 
lic mortar  is  the  only  kind  which  will  form  a  safe  construc- 
tion. The  wall  which  supplies  the  slope  of  the  excavation' 
should  be  carried  up  as  high  as  the  natural  surface  of  the 
ground ;  the  one  that  sustains  the  embankment  should  be 
built  up  to  the  surface  of  the  roadway ;  and  a  parapet- wall 
should  be  raised  upon  it,  to  secure  vehicles  from  accidents  in 
deviating  from  the  line  of  the  roadway. 

A  road  may  be  constructed  partly  in  excavation  and  partly 
in  embankment  along  a  rocky  ledge,  by  blasting  the  rock, 


416 


CIVIL  ENGINEEEING. 


when  the  inclination  of  the  natural  surface  is  not  greater  than 
one  perpendicular  to  two  base  ;  but  with  a  greater  inclination 
than  this,  the  whole  should  be  in  excavation. 


719.  There  are  examples  of  road  constructions,  in  localities 
like  the  last,  supported  on  a  framework,  consisting  of  hori- 
zontal pieces,  which  are  firmly  fixed  at  one  end  by  being  let 
into  holes  drilled  in  the  rock,  and  are  sustained  at  the  other 
by  an  inclined  strut  underneath,  which  rests  against  the  rock 
in  a  shoulder  formed  to  receive  it. 

720.  When  the  excavations  do  not  furnish  sufficient  earth 
for  the  embankments,  it  is  obtained  from  excavations  termed 
side-cuttings,  made  at  some  place  in  the  vicinity  of  the  em- 
bankment, from  which  the  earth  can  be  obtained  with  most 
economy. 

If  the  excavations  furnish  more  earth  than  is  required  for 
the  embankment,  it  is  deposited  in  what  is  termed  spoil-hank, 
on  the  side  of  the  excavation.  The  spoil-bank  should  be 
made  at  some  distance  back  from  the  side  slope  of  the  exca- 
vation, and  on  the  down-hill  side  of  the  top  surface ;  and 
suitable  drains  should  be  arranged  to  carry  off  any  water 
that  might  collect  near  it  and  affect  the  side  slope  of  the  ex- 
cavation. 

The  forms  to  be  given  to  side-cuttings  and  spoil-banks  will 
depend,  in  a  great  degree,  upon  the  locality :  they  should,  as 
far  as  practicable,  be  such  that  the  cost  of  removal  of  the 
earth  shall  be  the  least  possible. 

721.  Drainage.  A  system  of  thorough  drainage,  by  which 
the  water  that  filters  through  the  ground  will  be  cut  off  from 
the  soil  beneath  the  roadway,  to  a  depth  of  at  least  three  feet 
below  the  bottom  of  the  road-covering,  and  by  which  that 
which  falls  upon  the  surface  will  be  speedily  conveyed  off, 


Fig.  223. — Cross  eection  of  a  road  in  steep 


side-forming. 

A,  filling. 

B,  sustaining-wall  of  filling. 

C,  breast-wall  of  cutting. 

D,  parapet-wall  of  footpath. 


COMMON  EOADS. 


417 


before  it  can  filter  tlirongli  the  road-covering,  is  essential  to 
the  good  condition  of  a  road. 

The  surface-water  is  conveyed  off  by  giving  the  surface  of 
the  roadway  a  slight  transverse  convexity,  from  the  middle 
to  the  sides,  where  the  water  is  received  into  the  gutters,  or 
side-channels^  from  which  it  is  conveyed  by  underground 
aqueducts,  termed  culverts^  built  of  stone  or  brick  and  usually 
arched  at  top,  into  the  main  drains  that  communicate  with 
the  natural  water-courses.  This  convexity  is  regulated  by 
making  the  figure  of  the  profile  an  ellipse,  of  which  the  semi- 
transverse  axis  is  15  feet,  and  the  serhi-conjugate  axis  9  inches; 
thus  placing  the  middle  of  the  roadway  nine  inches  above  the 
bottom  of  the  side  channels.  This  convexity,  which  is  as  great 
as  should  be  given,  will  not  be  sufficient  in  a  flat  country  to 
keep  the  road-surface  dry ;  and  in  such  localities,  if  a  slight 
longitudinal  slope  cannot  be  given  to  the  road,  it  should  be 
raised,  when  practicable,  three  or  four  feet  above  the  general 
level ;  both  on  account  of  conveying  off  speedily  the  surface- 
water,  and  exposing  the  surface  better  to  the  action  of  the 
wind. 

To  drain  the  soil  beneath  the  roadway  in  a  level  country, 
ditches,  termed  oj^en  side  drains  (Fig.  224),  are  made  paral- 


Fig.  224.— Cross-section  of  broken-stone  road-covering. 

A,  road-surface. 

B,  side  channels. 
0,  footpath. 

D,  covered  drains,  or  culverts,  leading  from  side  channels  to  the  side  drains  E. 


lei  to  the  road,  and  at  some  feet  from  it  on  each  side.  The 
bottom  of  the  side  drains  should  be  at  least  three  feet  below 
the  road-covering ;  their  size  will  depend  on  the  nature  of  the 
soil  to  be  drained.  In  a  cultivated  country  the  side  drains 
should  be  on  the  field  side  of  the  fences. 

As  open  drains  would  be  soon  filled  along  the  parts  of  a 
road  in  excavation,  by  the  washings  from  the  side-slopes, 
covered  drains,  built  either  of  brick  or  stone,  must  be  substi- 
tuted for  them.  These  drains  (Fig.  225)  consist  simply  of  a 
flooring  of  flagging  stone,  or  of  brick,  with  two  side  walls  of 
27 


418  CIVIL  ENGINEERING. 

I 

rubble,  or  brick  masonry,  which  support  a  top  coverino;  of 
flat  stones,  or  of  brick,  with  open  joints,  of  about  half  an 
inch,  to  give  a  free  passage-way  to  the  water  into  the  drain. 
The  top  is  covered  with  a  layer  of  straw  or  brushwood ;  and 
clean  gravel,  or  broken  stone,  in  small  fragments,  is  laid  over 
this,  for  the  purpose  of  allowing  the  water  to  filter  freely 
through  to  the  drain,  without  carrying  w^ith  it  any  earth  or 
sediment,  which  might  in  time  accumulate  and  choke  it. 
The  width  and  height  of  covered  drains  will  depend  on  the 
materials  of  which  they  are  built,  and  the  quantity  of  water 
to  which  they  yield  a  passage. 


Fig.  225.— Cross-section  of  a  covered  drain. 

A,  drain. 

a,  a,  side  walls, 

6,  top  stones. 

c,  bottom  stones. 

d,  broken  stone  or  large  gravel  laid  over  brush. 


Besides  the  longitudinal  covered  drains  in  cuttings,  other 
drains  are  made  under  the  roadway  w^hich,  from  their  form, 
are  termed  cross  initre  drains.  Their  plan  is  in  shape  like 
the  letter  Y,  the  angular  point  being  at  the  centre  of  the 
road,  and  pointing  in  the  direction  of  its  ascent.  The  angle 
should  be  so  regulated  that  the  bottom  of  the  drain  shall  not 
have  a  greater  slope  along  either  of  its  branches,  than  one 
perpendicular  to  one  hundred  base,  to  preserve  the  masonry 
from  damage  by  the  current.  The  construction  of  mitre 
drains  is  the  same  as  the  covered  longitudinal  drains.  They 
should  be  placed  at  intervals  of  about  60  yards  from  each 
other. 

In  some  cases  surface  drains,  termed  catch-water  drains^ 
are  made  on  the  side  slopes  of  cuttings.  They  are  run  up 
obliquely  along  the  surface,  and  empty  directly  into  the  cross 
drains  which  convey  the  water  into  the  natural  water-courses. 

When  the  roadway  is  in  side-forming,  cross  drains  of  the 
ordinary  form  of  culverts  are  made  to  convey  the  water  from 
the  side  channels  and  the  covered  drains  into  the  natural 
water-courses.  They  should  be  of  sufficient  dimensions  to 
convey  off  a  large  volume  of  water,  and  to  admit  a  man  to 
pass  through  them  so  that  they  may  be  readily  cleared  out, 
or  even  repaired,  without  breaking  up  the  roadway  over 
them. 

The  only  drains  required  for  embankments  are  the  ordi- 


COMMON  EOADS. 


419 


naiT  side  channels  of  the  roadway,  with  occasional  culverts  to 
convey  the  water  from  them  into  the  natural  water-courses. 
Great  care  should  be  taken  to  prevent  the  surface-water  from 
running  down  the  side  slopes,  as  they  would  soon  be  w^ashed 
into  gullies  by  it. 

Vei-y  wet  and  marshy  soils  require  to  be  thoroughly  drained 
before  the  roadway  can  be  made  with  safety.  The  best 
system  that  can  be  followed  in  such  cases  is  to  cut  a  wide 
and  deep  open  main-drain  on  each  side  of  the  road,  to  con- 
vey the  water  to  the  natural  water-courses.  Covered  cross 
drains  should  be  made  at  frequent  intervals,  to  drain  the  soil 
under  the  roadway.  They  should  be  sunk  as  low  as  will  ad- 
mit of  the  water  running  from  them  into  the  main  drains, 
by  giving  a  slight  sloj^e  to  the  bottom  each  way  from  the 
centre  of  the  road  to  facilitate  its  flow. 

Independently  of  the  drainage  for  marshy  soils,  the}-  will 
require,  when  the  subsoil  is  of  a  spongy,  elastic  nature,  an 
artificial  bed  for  the  road  covering.  This  bed  may,,  in  some 
cases,  be  formed  by  simj^ly  removing  the  upper  stratum  to  a 
depth  of  several  feet,  and  supplying  its  place  with  well- 
packed  gravel,  or  any  soil  of  a  firm  character.  In  other  cases, 
when  the  subsoil  yields  readily  to  the  ordinary  pressure  that 
the  road-surface  must  bear,  a  bed  of  brushwood,  from  9  to  18 
inches  in  thickness,  must  be  formed  to  receive  the  soil  on 
w^hich  the  road-covering  is  to  rest.  The  brushwood  should  be 
carefully  selected  from  the  long  straight  slender  shoots  of  the 
branches  or  undergrow^th,  and  be  tied  up  in  bundles,  termed 
faschies^  from  9  to  12  inches  in  diameter,  and  from  10  to  20 
feet  long.  The  fascines  are  laid  in  alternate  layers  crosswise 
and  lengthwise,  and  the  layers  are  either  connected  by  pick- 
ets, or  else  the  withes,  with  which  the  fascines  are  bound,  are 
cut  to  allow  the  brushwood  to  form  a  uniform  and  compact  bed. 

This  method  of  securing  a  good  bed  for  structures  on  a 
weak  wet  soil  has  been  long  practised  in  Holland,  and  ex- 
perience has  fully  tested  its  excellence. 

722.  Road-coverings.  The  object  of  a  road-covering  being 
to  diminish  the  resistances  arising  from  collision  and  friction, 
and  thereby  to  reduce  the  force  of  traction  to  the  least  j)rac- 
ticable  amount,  it  should  be  composed  of  hard  and  durable 
materials,  laid  on  a  firm  foundation,  and  present  a  uniform, 
even  surface. 

The  material  in  ordinary  use  for  road-coverings  is  stone, 
either  in  the  shape  of  blocks  of  a  regular  form,  or  of  large 
round  pebbles,  termed  a  pavement^  or  broken  into  small  an- 
gular masses ;  or  in  the  form  of  gravel. 


420 


CIVIL  ENGINEERING. 


723.  Pavements.  The  pavements  in  most  general  nse  in 
our  country  are  constructed  of  rounded  pebbles,  known  as 
2>aving  stones,  varying  from  3  to  8  iuches  in  diameter,  which' 
are  set  in  ^form,  or  bed  of  clean  sand  or  gravel,  a  foot  or 
two  in  thickness,  which  is  laid  upon  the  natural  soil  excavated 
to  receive  tlie  form.  The  largest  stones  are  placed  in  the 
centre  of  the  roadway.  The  stones  are  carefully  set  in  the 
form,  in  close  contact  with  each  other,  and  are  then  firmly 
settled  by  a  heavy  rammer  until  their  tops  are  even  with  the 
general  surface  of  the  roadway,  which  should  be  of  a  slightly 
convex  shape,  having  a  slope  of  about  ^  from  the  centre 
each  way  to  the  sides.  After  the  stones  are  driven,  the  road- 
surface  is  covered  with  a  layer  of  clean  sand,  or  fine  gravel, 
two  or  three  inches  in  thickness,  which  is  gradually  worked 
in  between  the  stones  by  the  combined  action  of  the  travel 
over  the  pavement  and  of  the  weather. 

The  defects  of  pebble  pavements  are  obvious,  and  con- 
firmed by  experience.  The  form  of  sand  or  gravel,  as 
usually  made,  is  not  sufiiciently  firm  ;  it  should  be  made  in 
separate  layers  of  about  4  inches,  each  layer  being  moistened 
and  well  settled  either  by  ramming,  or  passing  a  heavy  roller 
over  it.  Upon  the  form  prepared  in  this  way  a  layer  of 
loose  material  of  two  or  three  inclies  in  thickness  may  be 
placed  to  receive  the  ends  of  the  paving  stones.  From  tlie 
form  of  the  pebbles,  the  resistance  to  traction  arising  from 
collision  and  friction  is  very  great. 

Pavements  termed  stone  trmnways  have  been  tried  in  some 
of  the  cities  of  Europe,  both  for  light  and  heavy  traftic. 
They  are  formed  by  laying  two  lines  of  long  stone  blocks  for 
the  wheels  to  run  on,  with  a  pavement  of  pebble  for  the  horse- 
track  between  the  wheel-tracks.  In  crowded  tlioronglifares 
tramwa^^s  offer  but  few  if  any  advantages,  as  it  is  impi'acti cable 
to  confine  the  vehicles  to  them,  and  when  exposed  to  heavy 
traffic  they  wear  into  ruts.  The  stone  blocks  should  be  care- 
fully laid  on  a  very  firm  bottoming,  and  particular  attention 
is  requisite  to  prevent  ruts  from  forming  between  the  blocks 
and  the  pe])ble  pavement. 

Stone  suitable  for  pavements  should  be  hard  and  tough,  and 
not  wear  smooth  mider  the  action  to  which  it  is  exposed. 
Some  varieties  of  granite  have  been  found  in  England  to 
furnish  the  best  paving  blocks.  In  France,  a  very  fine-grained 
compact  gray  sandstone  of  a  bluish  cast  is  mostly  in  use  for 
the  same  purpose,  but  it  wears  quite  smooth. 

The  sand  used  for  forms  should  be  clean  and  free  from  peb- 
bles and  gravel  of  a  larger  grain  than  about  two-tenths  of  an 


COIUMON  ROADS. 


421 


inch.  The  form  should  be  made  by  moistening  the  sand,  and 
compressing  it  in  layers  of  about  four  inches  in  thickness, 
either  by  ramming,  or  by  passing  over  each  layer  several  times 
a  heavy  iron  roller.  Upon  the  top  layer  about  an  inch  of 
loose  sand  may  be  spread  to  receive  the  blocks ;  the  joints 
between  which,  after  they  are  placed,  should  be  carefully 
filled  with  sand. 

The  sand  form,  when  carefully  made,  presents  a  very  firm 
and  stable  foundation  for  the  pavement. 

Wooden  pavements,  formed  of  blocks  of  wood  of  various 
shapes,  have  been  tried  in  England  and  several  of  our  cities 
within  the  last  few  years,  and  notwithstanding  they  decay  in 
a  few  years,  yet  they  are  extensively  used  in  many  of  our  large 
cities.  The  travel  upon  them  is  so  free  from  noise,  and  the 
surface  is  so  smooth,  that,  on  those  streets  where  the  haulage 
of  heavy  articles  is  not  excessive,  many  property  holders  prefer 
to  renew  a  wooden  pavement  every  eight  or  ten  years,  than  be 
annoyed  w^th  the  noise  and  the  roughness  of  stone  pavements. 
They  are  especially  desirable  upon  those  streets  which  are  oc- 
cupied by  residences. 

Asphaltic  pavements  have  undergone  a  like  trial,  and 
have  been  found  to  fail  after  a  few  years'  service.  This 
material  is  farther  objectionable  as  a  pavement  in  cities  where 
the  pavements  and  sidewalks  have  frequently  to  be  disturbed 
for  the  purposes  of  repairing,  or  laying  down  sewers,  water- 
pipes,  and  other  necessary  conveniences  for  a  city. 

The  best  system  of  pavement  is  that  which  has  been 
partially  put  in  practice  in  some  of  the  commercial  cities  of 
England,  the  idea  of  which  seems  to  have  been  taken  from  the 
excellent  military  roads  of  the  Romans,  vestiges  of  which  re- 
main at  the  present  day  in  a  good  state. 


Fig.  226. — Paved  road-covering. 

A,  pavement. 
C,  curb-stone. 

B,  flagging  of  side-walk. 


In  constructing  this  pavement,  a  bed  (Fig.  226)  is  first  pre- 
pared, by  removing  the  surface  of  the  soil  to  the  depth  of  a 
foot  or  more,  to  obtain  a  firm  stratum ;  the  surface  of  this  bed 
receives  a  very  slight  convexity,  of  about  two  inches  to  ten 
feet,  from  the  centre  to  the  sides  of  the'  roadway.  If  the  soil 
is  oi  a  soft  clayey  nature,  into  which  small  fragments  of 


422 


CIVIL  ENGmEEEING. 


broken  stone  would  be  easily  worked  by  the  wheels  of  vehicles, 
it  should  be  ex(;avated  a  foot  or  two  deeper  to  receive  a  form 
of  sand,  or  of  clean  fine  gravel.  On  the  surface  of  the  bed 
thus  prepared,  a  layer  of  small  broken  stone,  four  inches 
thick,  is  laid;  the  dimensions  of  these  fragments  should  not 
be  greater  than  two  and  a  half  inches  in  any  direction;  the 
road  is  then  opened  to  vehicles  until  this  first  layer  becomes 
perfectly  compact ;  care  being  taken  to  fill  up  any  ruts  with 
fresh  stone,  in  order  to  obtain  a  uniform  surface.  A  second 
layer  of  stone,  of  the  same  thickness  as  the  first,  is  then  laid 
on,  and  treated  in  the  same  manner;  and  finally  a  third  layer. 
When  the  third  layer  has  become  perfectly  compact,  and  is 
of  a  uniform  surface,  a  layer  of  fine  clean  gravel,  two  and  a 
half  inches  thick,  is  spread  evenly  over  it  to  receive  the 
paving  stones.  The  blocks  of  stone  are  of  a  square  shape,  and 
of  different  sizes,  according  to  the  nature  of  the  travelling 
over  the  pavement.  The  largest  size  are  ten  inches  thick, 
nine  inches  broad,  and  twelve  inches  long;  the  smallest  are 
six  inches  thick,  five  inches  broad,  and  ten  inches  long. 
Each  block  is  carefully  settled  in  the  form,  by  means  of  a 
heavy  beetle;  it  is  then  removed  in  order  to  cover  the  side  of 
the  one  against  which  it  is  to  rest  with  hydraulic  mortar ; 
this  being  done,  the  block  is  replaced,  and  properly  adjusted. 
The  blocks  of  the  different  courses  across  the  roadway  should 
break  joints.  The  surface  of  the  road  is  convex;  the  con- 
vexity being  determined  by  making  the  outer  edges  six  inches 
lower  than  the  middle,  for  a  width  of  thirty  feet. 

This  system  of  pavement  fulfils  in  the  best  manner  all  the 
requisites  of  a  good  road-covering,  presenting  a  hard  even 
surface  to  the  action  of  the  wheels,  and  reposing  on  a  firm 
bed  formed  by  the  broken-stone  bottoming.  The  mortar- 
joints,  so  long  as  they  remain  tight,  will  effectually  prevent 
the  penetration  of  water  beneath  the  pavement ;  but  it  is 
probable,  from  the  effect  of  the  transit  of  heavily-laden 
vehicles,  and  from  the  expansion  and  contraction  of  the  stone, 
which  in  our  climate  is  found  to  be  very  considerable,  that 
the  mortar  would  soon  be  crushed  and  washed  out. 

In  France,  and  in  many  of  the  large  cities  of  the  continent, 
\hQ  pavements  are  made  with  hlochs  of  rough  stone  of  a  cxibi- 
cal  form  rneasuring  between  eight  and  nine  inches  along  the 
edge  of  the  cube.  These  are  laid  on  a  form  of  sand  of  only  a 
few  inches  thick  when  the  soil  beneath  is  firm  ;  but  in  bad  soils 
the  thickness  is  increased  to  from  six  to  twelve  inches.  The 
transversal  joints  are  usually  continuous,  and  those  in  the 
direction  of  the  axis  of  the  road  break  joints.    In  some  cases 


COMMON  ROADS. 


423 


the  blocks  are  so  laid  that  the  joints  make  an  angle  of  45°  with 
the  axis  of  the  roadway,  one  set  being  continuous,  the  other 
breaking  joints  with  them.  By  this  arrangement  of  the 
joints,  it  is  said  that  the  wear  upon  the  edges  of  the  blocks, 
by  which  the  upper  surface  soon  assumes  a  convex  shape,  is 
diminished.  It  has  been  ascertained  by  experience  that  the 
wear  upon  the  edges  of  the  blocks  is  greatest  at  the  joints 
which  run  transversely  to  the  axis  when  the  blocks  are  laid  in 
the  usual  manner.  From  the  experiments  of  M.  Morin,  to 
ascertain  the  influence  of  the  shape  of  stone  blocks  on  the 
force  of  traction,  it  was  found  that  the  resistance  offered  by  a 
pavement  of  blocks  averaging  from  five  to  six  inches  in 
breadth,  measured  in  the  direction  of  the  axis  of  the  road- 
way, and  about  nine  inches  in  length,  was  less  than  in  one  of 
cubical  blocks  of  the  ordinary  size. 

Pavements  in  cities  must  be  accompanied  by  sidewalhs 
and  crossing-jplaGes  for  foot-jpassengers.  The  sidewalks  are 
made  of  large  flat  flagging-stone,  at  least  two  inches  thick, 
laid  on  a  form  of  clean  gravel  well  rammed  and  settled.  The 
width  of  the  sidewalks  will  depend  on  the  street  being  more 
or  less  frequented  by  a  crowd.  It  would,  in  all  cases,  be  well 
to  have  them  at  least  twelve  feet  wide  ;  they  receive  a  slope, 
or  pitch,  of  one  inch  to  ten  feet,  towards  the  pavement,  to 
convey  the  surface-water  to  the  side  channels.  The  pavement 
is  separated  from  the  sidewalk  by  a  row  of  long  slabs  set  on 
their  edges,  termed  curbstones^  which  confine  both  the  fiag- 
ging  and  paving  stones.  The  curb-stones  form  the  sides  of 
the  side  channels,  and  should  for  this  purpose  project  six 
inches  above  the  outside  paving  stones,  and  be  sunk  at  least 
four  inches  below  their  top  surface ;  they  should,  moreover, 
be  flush  with  the  upper  surface  of  the  sidewalks,  to  allow  the 
water  to  run  over  into  the  side  channels,  and  to  prevent  acci- 
dents which  might  otherwise  happen  from  their  tripping 
persons  passing  in  haste. 

The  crossings  should  be  from  four  to  six  feet  wide,  and  be 
slightly  raised  above  the  general  surface  of  the  pavement,  to 
keep  them  free  from  mud. 

724.  Broken-stone  Road-covering.  The  ordinary  road- 
covering  for  common  roads,  in  use  in  this  country  and  Eu- 
rope, is  formed  of  a  coating  of  stone  broken  into  small  frag- 
ments, which  is  laid  either  upon  the  natural  soil,  or  upon  a 
paved  bottoming  of  small  irregular  blocks  of  stone.  In 
England  these  two  systems  have  their  respective  partisans ; 
the  one  claiming  the  superiority  for  road-coverings  of  stone 
broken  into  small  fragments,  a  method  brought  into  vogue 


424 


CIVIL  ENGINEERING. 


some  years  since  by  Mr.  McAdara,  from  whom  these  roads 
have  been  termed  7aacadamized ;  the  other  being  the  plan 
pursued  by  Mr.  Telford  in  the  great  national  roads  construct- 
ed in  Great  Britain  within  about  the  same  period. 

The  subject  of  road-making  has  within  the  last  few  years 
excited  renewed  interest  and  discussion  among  engineers 
in  France ;  the  conclusion,  drawn  from  experience,  there 
generally  adopted  is,  that  a  covering  alone  of  stone  broken 
into  small  fragments  is  sufficient  under  the  heaviest  traffic 
and  most  frequented  roads.  Some  of  the  French  engineers 
recommend,  in  very  yielding  clayey  soils,  that  either  a  paved 
bottoming  after  Telford's  method  be  resorted  to,  or  that  the 
soil  be  well  compressed  at  the  surface  before  placing  the 
i-oad-covering. 

The  paved  bottom  road-covering  on  Telford's  plan  (Fig. 
225),  is  formed  by  excavating  the  surface  of  the  ground  to  a 
suitable  depth,  and  preparing  the  form  for  the  pavement  with 
tlie  precautions  as  for  a  common  pavement.  Blocks  of  stone 
of  an  irregular  pyramidal  shape  are  selected  for  the  pave- 
ment, which,  for  a  roadway  30  feet  in  width,  should  be  seven 
inches  thick  for  the  centre  of  the  road,  and  three  inches 
thick  at  the  sides.  The  base  of  each  block  should  not 
measure  more  than  five  inches,  and  the  top  not  less  than  four 
inches. 

The  blocks  are  set  by  the  hand,  with  great  care,  as  closely 
in  contact  at  their  bases  as  practicable;  and  blocks  of  a 
suitable  size  are  selected  to  give  the  surface  of  the  pavement 
a  slightly  convex  shape  from  the  centre  outwards.  The 
spaces  between  the  blocks  are  filled  with  cliippings  of  stone 
compactly  set  with  a  small  hammer. 

A  layer  of  broken  stone,  four  inches  thick,  is  laid  over  this 
pavement,  for  a  width  of  nine  feet  on  each  side  of  the  centre  ; 
no  f  j-agment  of  this  layer  should  measure  over  two  and  a  half 
inches  in  any  direction.  A  layer  of  broken  stone  of  smaller 
dimensions,  or  of  clean  coarse  gravel,  is  spread  over  the  wings 
to  the  same  depth  as  the  centre  layer. 

The  road-covering,  thus  prepared,  is  thrown  open  to  vehi- 
cles until  the  upper  layer  has  become  perfectly  compact ; 
care  having  been  taken  to  fill  in  the  ruts  with  fresh  stone, 
in  order  to  obtain  a  uniform  surface.  A  second  layer,  about 
two  inches  in  depth,  is  then  laid  over  the  centre  of  the  road- 
way ;  and  the  wings  receive  also  a  layer  of  new  material  laid 
on  to  a  sufficient  thickness  to  make  the  outside  of  the  roadway 
nine  inches  lower  than  the  centre,  by  giving  a  slight  convexi- 
ty to  the  surface  from  the  centre  outwards.    A  coating  of 


COI^mON  ROADS. 


425 


clean  coarse  gravel,  one  inch  and  a  half  thick,  termed  a 
hiiiding,  is  spread  over  the  surface,  and  the  road-covering  is 
then  ready  to  be  thrown  open  to  travelling. 

The  stone  used  for  the  pavement  may  be  of  an  inferior 
quality,  in  hardness  and  strength,  to  that  placed  at  the  surface, 
as  it  is  but  little  exposed  to  the  wear  and  tear  occasioned  by 
travelling.  The  surface-stone  should  be  of  the  hardest  kind 
that  can  be  procured.  The  gravel  binding  is  laid  over  the 
surface  to  facilitate  the  travelling,  whilst  the  under  stratum 
of  stone  is  still  loose ;  it  is,  however,  hurtful,  as,  by  working 
in  between  the  broken  stones,  it  prevents  them  from  setting 
as  compactly  as  they  would  otherwise  do. 

If  the  roadway  cannot  be  paved  the  entire  width,  it  should, 
at  least,  receive  a  pavement  for  the  width  of  nine  feet  on 
each  side  of  the  centre.  The  wings,  in  this  case,  may  be 
formed  entirely  of  clean  gravel,  or  of  chippings  of  stone. 
'  For  roads  which  are  not  much  travelled,  like  the  ordinary 
cross  roads  of  the  country,  the  pavement  will  not  demand  so 
much  care ;  but  may  be  made  of  any  stone  at  hand,  broken 
into  fragments  of  such  dimensions  that  no  stone  shall  weigh 
over  four  pounds.  The  surface-coating  may  be  formed  in  the 
manner  just  described. 

725.  In  forming  a  road-covering  of  hroken  stone  alone, 
the  bed  for  the  covering  is  arranged  in  the  same  manner  as 
for  the  paved  bottoming  :  a  layer  of  the  stone,  four  inches  in 
thickness,  is  carefully  spread  over  the  bed,  and  the  road  is 
thrown  open  to  vehicles,  care  being  taken  to  fill  the  ruts,  and 
preserve  the  surface  in  a  uniform  state  until  the  layer  has  be- 
come compact ;  successive  layers  are  laid  on  and  treated  in 
the  same  manner  as  the  first,  until  the  covering  has  received 
a  thickness  of  about  twelve  inches  in  the  centre,  with  the 
ordinary  convexity  at  the  surface. 

726.  Gravel  Roads.  Where  good  gravel  can  be  procured 
the  road-covering  may  be  made  of  this  material,  which  should 
be  well  screened,  and  all  pebbles  found  in  it  over  two  and  a 
half  inches  in  diameter,  should  be  broken  into  fragments  of 
not  greater  dimensions  than  these.  A  firm  level  form  having 
been  prepared,  a  layer  of  gravel,  four  inches  in  thickness,  is 
laid  on,  and,  when  this  has  become  compact  from  the  travel, 
successive  layers  of  about  three  inches  in  thickness  are  laid 
on  and  treated  like  the  first,  until  the  covering  has  received 
a  thickness  of  sixteen  inches  in  the  centre  and  the  ordinary 
convexity. 

The  Superintending  Engineer  of  Central  Park,  of  ISTew 
York  City,  Mr.  W.  H.  Grant,  made  experiments  upon  Telford, 


426 


CIYIL  ENGmEERING. 


McAdam,  and  gravel  roads  in  the  Park,  and  he  came  to  the 
conchision  that  the  gravel  roads,  as  there  constructed,  were 
better  for  tlie  purposes  of  park  roads  than  either  of  the 
others.  {Journal  of  the  FranJdin  Institute,  .  Yol.  84, 
p.  233.) 

The  gravel  roads  which  were  constructed  by  him  had  a 
rubble,  or  broken-stone  foundation,  over  which  was  passed  a 
very  heavy  roller  ;  and  upon  whicli  was  placed  layers  of  gravel 
which  were  thoroughly  rolled.     In  some  cases  screened 

f ravel  was  used,  and  in  others  gravel  directly  from  the  bed. 
*aved  foundations  for  receiving  the  gravel  make  the  road 
much  more  durable,  although  the  original  cost  is  considerably 
increased  thereby.  Roads  of  this  kind,  which  are  constantly 
used,  should  be  frequently  repaired,  and  the  additional  layers 
of  gravel  should  be  thoroughly  pressed  with  a  heavy  roller. 
For  detailed  information,  see  Journal  of  the  Franldin  Insti- 
tute, 1867.  Yol.  83,  pp.  100,  153,  233,  297  and  391,  and  Yol. 
84,  pp.  233  and  311. 

727.  As  has  been  already  stated,  the  French  civil  engineers 
do  not  regard  a  paved  bottoming  as  essential  for  broken-stone 
road-coverings,  except  in  cases  of  a  very  heavy  traffic,  or  where 
the  substratum  of  the  road  is  of  a  very  yielding  character. 
They  also  give  less  thickness  to  the  road-covering  than  the 
English  engineers  of  Telford's  school  deem  necessary  ;  allow- 
ing not  more  than  six  to  eight  inches  to  road-coverings  for 
light  traffic,  and  about  ten  inches  only  for  the  heaviest  traffic. 

If  the  soil  upon  which  the  road-covering  is  to  be  placed  is 
not  dry  and  firm,  they  compress  it  hy  rolling,  which  is  done 
by  passing  over  it  several  times  an  iron  cylinder,  about  six 
feet  in  diameter,  and  four  feet  in  length,  the  weight  of  which 
can  be  increased,  by  additional  weights,  from  six  thousand  to 
about  twenty  thousand  pounds.  The  road  material  is  placed 
upon  the  bed,  when  well  compressed  and  levelled,  in  layers 
of  about  four  inches,  each  layer  being  compressed  by  passing 
the  cylinder  several  times  over  it  before  a  new  one  is  laid  on. 
If  the  operation  of  rolling  is  performed  in  dry  weather,  the 
layer  of  stone  is  watered,  and  some  add  a  thin  layer  of  clean 
sand,  from  four  to  eight  tenths  of  an  inch  in  thickness,  over 
each  layer  before  it  is  rolled,  for  the  purpose  of  consolidating 
the  surface  of  the  layer,  by  filling  the  voids  between  tlie 
broken-stone  fragments.  After  the  surface  has  been  well 
consolidated  by  rolling,  the  road  is  thrown  open  for  travel, 
and  all  ruts  and  other  displacement  of  the  stone  on  the  sur- 
face are  carefully  repaired,  by  adding  fresh  material,  and 
levelling  the  ridges  by  ramming. 


COMMON  ROADS. 


427 


Great  importance  is  attached  by  tlie  French  engineers  to 
the  nse  of  the  iron  cyHnder  for  compressing  the  materials  of 
a  new  road,  and  to  minute  attention  to  daily  repairs.  It  is 
stated  that  by  the  use  of  the  cylinder  the  road  is  presented 
at  once  in  a  good  travelling  condition ;  the  wear  of  the  ma- 
terials is  less  than  by  the  old  method  of  gradually  consoli- 
dating them  by  the  travel ;  the  cost  of  repairs  during  the 
first  3'ear  is  diminished  ;  it  gives  to  the  road-covering  a  more 
nniform  thickness,  and  admits  of  its  being  thinner  than  in  the 
usual  metliod. 

The  iron  roller  is  now  moA^ed  by  a  locomotive,  to  which  it 
is  attached  by  a  suitable  gearing,  that  admits  of  reversing,  so 
as  to  travel  backward  and  forward  over  the  road  surface. 

728.  Asphaltic  Roadways  and  Sidewalks.  In  pre- 
paring roadways  with  an  asphaltic  surface,  the  ground  or 
subsoil  is  first  made  level  crosswise,  and  very  compact,  by 
rolling  it  with  a  heavy  cylinder.  Upon  this  a  bed  of  hy- 
draulic concrete,  consisting  of  one  part  in  volume  of  hy- 
draulic mortar,  to  two  and  a  quarter  parts  in  volume  of 
gravel,  is  laid  to  the  thickness  of  two  and  a  half  inches. 
This  foundation  is  allowed  to  become  perfectly  hard  and  dry 
before  the  asphalt  is  laid  over  it. 

The  asphaltic  rock  reduced  to  powder  by  the  ordinary  pro- 
cess is  uniformly  spread  over  the  concrete  bed,  the  surface  of 
which  should  be  tiioroughly  dry  before  receiving  the  mastic^, 
to  the  depth  of  two  to  two  and  a  half  inches.  This  will  pro- 
duce a  layer  of  packed  material  varying  from  one  and  three- 
quarters  to  two  inches  in  thickness. 

The  packing  is  done  with  hot  irons  or  pestles,  worked  by 
hand,  and  applied  lightly,  so  as  to  produce  a  uniform  smooth 
surface.  After  the  upper  bed  is  compressed  in  this  manner 
to  a  proper  thickness,  a  thin  coat  of  fine  dry  powder,  the 
siftings  of  earth  or  of  mineral  coal  ashes,  is  spread  over  the 
surface  to  fill  up  inequalities,  and  the  surface  is  again 
smoothed  over  by  a  flat-iron,  heated  nearly  to  a  red  heat ; , 
and,  whilst  the  asphalt  is  still  hot,  it  is  rolled  with  polished 
iron  rollers,  the  lighter,  weighing  four  hundred  and  forty 
pounds,  being  first  applied,  and  then  a  heavier,  weighing, 
three  thousand  pounds. 

In  recommencing  work  on  an  unfinished  portion,  the  part 
to  which  the  fresh  material  is  to  be  joined  is  first  thoroughly 
cleansed  from  dust,  and  hot  asphalt  poured  over  it. 

For  sidewalks  the  asphaltic  rock  is  reduced  to  a  powder, 
either  by  crushing  it  under  rollers  or  by  roasting;  this  is 
then  sifted  through  wire  gauze,  with  meshes  of  one-tenth  of 


428 


CIVIL  ENGINEERING. 


an  inch.  This  powder  is  thoroughly  incorporated  with  hot 
mineral  tar,  in  the  usual  way,  in  the  proportions  of  about 
three  hundred  and  thirty  pounds  of  tar  to  four  thousand  four 
hundred  pounds  of  powder.  This  mixture,  termed  mastic, 
r  can  be  cast  into  moulds  of  suitable  size  and  kept  for  use. 

To  one  hundred  pounds  of  this  mixture  five  or  six  pounds 
of  mineral  tar  are  added.  A  portion,  about  three  per  cent, 
of  the  mastic,  of  the  mineral  tar  is  first  heated  in  an  iron 
cylinder,  and  then  one-third  of  the  mastic  thoroughly  incor- 
porated with  it  by  stirring  with  an  iron  rod,  one  per  cent, 
more  of  the  tar  is  then  added,  and  next  another  third  of  the 
mastic,  and  the  remaining  portions  are  stirred  in  in  like 
manner.  When  the  w^hole  is  melted  one-half  the  gravel  is 
stirred  in,  and  then  the  remaining  half  in  the  same  way. 

In  warm  climates  the  mixture  may  receive  a  larger  dose  of 
gravel. 

When  the  subsoil  is  compact  and  dry  a  layer  of  concrete 
of  one  inch  and  a  half  in  thickness  is  spread  over  it,  and 
covered  by  a  layer  of  mortar  half  an  inch  thick ;  and  over 
this,  when  thoroughly  dry,  a  coat  of  one  inch  and  six-tenths 
of  the  prepared  mastic  concrete. 

When  the  soil  is  not  hard,  it  should  be  rammed  or  rolled 
to  make  it  so  before  receiving  tiie  hydraulic  concrete,  which, 
in  this  case,  is  three  inches  and  a  half  thick,  the  other  two 
courses  being  the  same  as  before. 

Tlie  mastic,  whilst  hot,  is  spread  uniformly  with  wooden 
trowels  over  the  mortar  bed ;  and  before  it  has  cooled  fine 
sand  is  sifted  over  the  surface. 

In  some  cases,  instead  of  a  bed  of  hydraulic  concrete  and 
mortar  to  receive  the  mastic  concrete,  one  of  hot  gravel, 
mixed  up  with  a  small  dose  of  mineral  tar,  is  laid,  and  over 
this  a  layer  of  concrete  mastic,  formed  of  the  fine  siftings  of 
mineral  coal  ashes,  mixed  up  with  heated  mineral  tar,  is  laid 
to  form  the  top  coating.  This,  in  like  manner,  may  receive 
a  sifting  of  fine  sand.  Rollers  are  used  in  this  case  to  give 
compactness  to  the  bed  and  the  upper  layer. 

729.  Materials  and  Repairs.  The 'materials  for  broken- 
stone  roads  should  be  hard  and  durable.  For  the  bottom 
layer  a  soft  stone,  or  a  mixture  of  hard  and  soft,  may  be 
used,  but  on  the  surface  none  but  the  hardest  stone  will  with- 
stand the  action  of  the  wlieels.  The  stone  should  be  care- 
fully broken  into  fragments  of  nearly  as  cubical  a  form  as 
practicable,  and  be  cleansed  from  dirt  and  of  all  very  small 
fragments.  The  broken  stone  should  be  kept  in  depots  at 
convenient  points  along  the  line  of  the  road  for  repairs. 


COMMON  ROADS. 


429 


Too  great  attention  cannot  be  bestowed  upon  keeping  the 
road-surface  free  from  an  accumulation  of  mud  and  even 
of  dust.  It  should  be  constantly  cleaned  by  scraping  and 
sweeping.  The  repairs  should  be  daily  made  by  adding  fresh 
material  upon  all  points  where  hollows  or  ]"uts  commence  to 
form.  It  is  recommended  by  some  that  when  fresh  material 
is  added,  the  surface  on  which  it  is  spread  should  be  broken 
with  a  pick  to  the  depth  of  half  an  inch  to  an  inch,  and  the 
fresh  material  be  well  settled  by  ramming,  a  small  quantity 
of  clean  sand  being  added  to  make  the  stone  pack  better. 
When  not  daily  repaired  by  persons  whose  sole  business  it  is 
to  keep  the  road  in  good  order,  general  repairs  should  be 
made  in  the  months  of  October  and  April,  by  removing  all 
accumulations  of  mud,  cleaning  out  the  side  channels  and 
other  drains,  and  adding  fresh  material  where  requisite. 

The  importance  of  keeping  the  road-surface  at  all  times 
free  from  an  accumulation  of  mud  and  dust,  and  of  preserv- 
ing the  surface  in  a  uniform  state  of  evenness,  by  the  daily 
addition  of  fresh  material,  wherever  the  wear  is  sufficient  to 
call  for  it,  cannot  be  too  strongly  insisted  upon.  Without 
this  constant  supervision,  the  best  constructed  road  will,  in  a 
short  time,  be  unlit  for  travel,  and  with  it  the  weakest  may  at 
all  times  be  kept  in  a  tolerably  fair  state. 

730.  Cross  Dimensions  of  Roads.  A  road  thirty  feet  in 
width  is  amply  sufficient  for  the  carriage-way  of  the  most  fre- 
quented thoroughfares  between  cities.  A  width  of  forty,  or 
even  sixty  feet,  may  be  given  near  cities,  where  the  greater 
part  of  the  transportation  is  effected  by  land.  For  cross  roads 
and  others  of  minor  importance,  the  width  may  be  reduced 
according  to  the  nature  of  the  case.  The  width  should  be 
at  least  sufficient  to  allow  two  of  the  ordinary  carriages  of 
the  country  to  pass  each  other  with  safety.  In  all  cases,  it 
should  be  borne  in  mind  that  any  unnecessary  width  increases 
both  the  lirst  cost  of  construction,  and  the  expense  of  annual 
repairs. 

Yery  wide  roads  have,  in  some  cases,  been  used,  the  centre 
part  only  receiving  a  road-covering,  and  the  wings,  termed 
summer  roads,  being  formed  on  the  natural  surface  of  the 
subsoil.  The  object  of  this  system  is  to  relieve  the  road-cov- 
ering from  the  Avear  and  tear  occasioned  by  the  lighter  kind 
of  vehicles  during  the  summer,  as  the  wings  present  a  more 
pleasant  surface  for  travelling  in  that  season..  But  little  is 
gained  by  this  system  under  this  point  of  view  ;  and  it  has 
the  inconvenience  of  forming  during  the  winter  a  large 
quantity  of  mud,  which  is  very  injurious  to  the  road-covering. 


430 


CIVIL  ENGINEEKING. 


There  should  be  at  least  one  foot-path,  from  five  to  six  feet 
wide,  and  not  more  than  nine  inches  higher  than  the  bottom 
of  the  side  channels.  The  surface  of  the  foot-path  should 
have  a  pitch  of  two  inches,  towards  the  side  channels,  to 
convey  its  surface-water  into  them.  AVlien  the  natural  soil  is 
firm  and  sandy,  or  gravelly,  its  surface  will  serve  for  the  foot 
path;  but  in  other  cases  the  natural  soil  must  be  thrown  out 
to  a  depth  of  six  inches,  and  the  excavation  be  filled  with  fine 
clean  gravel. 

To  prevent  the  foot-path  from  being  damaged  by  the  cur- 
rent of  water  in  the  side  channels,  its  slide  slope,  next  to  the 
side  channel,  must  be  protected  by  a  facing  of  good  sods,  or 
of  dry  stone. 

As  it  is  of  the  first  importance,  in  keeping  the  road- way  in 
a  good  travelling  state,  that  its  surface  should  be  kept  dry,  it 
will  be  necessary  to  remote  fi-om  it,  as  far  as  practicable,  all 
objects  that  might  obstruct  the  action  of  the  wind  and  the 
sun  on  its  surface.  Fences  and  hedges  along  the  road  should 
not  be  higher  than  five  feet;  and  no  trees  should  be  sufi^ered 
to  stand  on  the  road-side  of  the  side-drains,  for  independently 
of  shading  the  road-way,  their  roots  would  in  time  throw  up 
the  road-covering. 

731.  Piank-Roads.  Plank-roads  were  very  popular  a  few 
years  since.  The  road  was  carefully  graded,  then  stringers 
— one  on  each  side — were  imbedded  in  the  eai'th,  and  upon 
these  were  laid  planks,  three  or  four  inches  thick,  forming  a 
continuous  floor.  When  the  planks  are  new  and  well  laid 
this  makes  a  veiy  agreeable  road  for  haulage  and  for  pleasure 
rides,  but  when  the  planks  become  worn  and  displaced  it 
makes  a  very  disagreeable  road.  As  a  general  thing  they 
have  been  abandoned,  except  in  certain  localities  where  they 
are  maintained  on  account  of  peculiar  circumstances.  A 
good  gravel  road  has  been  found  to  be  more  profitable,  and 
in  the  long  run  makes  a  much  better  road.  Many  plank- 
roads  have  been  changed  to  McAdam  or  to  Telford  roads. 

II. 

RAILWAYS. 

732.  A  railway^  or  railroad^  is  a  ti-ack  for  the  wheels  of 
vehicles  to  run  on,  which  is  formed  of  iron  bars  placed  in 
two  pai-allel  lines  and  resting  on  firm  supports. 

733.  Rails.  The  iron  ways  first  laid  down,  and  termed 
tramways,  ^vere  made  of  narrow  iron  plates,  cast  in  short 


RAILWAYS. 


431 


lengths,  with  an  upright  flanch  on  the  exterior  to  confine  the 
wheel  within  the  track.  The  plates  were  found  to  be  de- 
licient  in  strength,  and  were  replaced  bj  others  to  which  a 
vertical  rib  was  added  under  the  plate.  This  rib  was  of  uni- 
form breadth,  and  of  the  shape  of  a  semi-ellipse  in  elevation. 
This  form  of  tramway,  although  superior  in  strength  to  the 
first,  was  still  found  not  to  work  well,  as  the  mud  which  ac- 
cumulated between  the  flanch  and  the  surface  of  the  plate 
presented  a  considerable  resistance  to  the  force  of  traction. 
To  obviate  this  defect,  iron  bars  of  a  semi-elliptical  shape  in 

Fig.  227 — Eepresents  a  cross-section  a,  of  the  fisli-bellied 
rail  of  the  Liverpool  and  Manchester  Railway,  and  the 
method  in  which  it  is  secured  to  its  chair.  The  rail  is 
foi-med  with  a  slight  projection  at  bottom,  which  fits 
into  a  corresponding  notch  in  the  side  of  the  chair  b. 
An  iron  wedsre  c  is  inserted  into  a  notch  on  the  opposite 
side  of  the  chair,  and  confines  the  rail  in  its  place. 


elevation,  which  received  the  name  of  edge-rails^  were  sub- 
stituted for  the  plates  of  the  tramway.  The  cross-sections  of 
these  rails  are  of  the  form  shown  in  Fig.  227,  the  top  surface 
being  slightly  convex,  and  sufficientl}^  broad  to  preserve  the 
tire  of  the  wheel  from  wearing  unevenly.  This  change  in 
the  form  of  the  rail  introduced  a  corresponding  one  in  the 
tires  of  the  wheels,  which  were  made  with  a  flanch  on  the 
interior  to  confine  them  within  the  rails  of  the  track. 

The  cast-iron  edge-rail  was  found  upon  trial  to  be  subject 
to  many  defects,  arising  from  the  nature  of  the  material. 
As  it  was  necessary  to  cast  the  rails  in  short  lengths  of  three 
or  four  feet,  the  tract  presented  a  number  of  joints,  which 
rendered  it  extremely  difiicult  to  preserve  a  uniform  surface. 
The  rails  were  found  to  break  readily,  and  the  surface  upon 
which  the  wheels  ran  wore  unevenly.  These  imperfections 
finally  led  to  the  substitution  of  wrought  iron  for  cast  iron. 

734.  The  wrought-iron  rails  first  brought  into  use  received 
nearly  the  same  shape  in  cross-section  and  elevation  as  the 
cast-iron  rail.  They  were  formed  by  rolling  them  out  in  a 
rolling-mill  so  arranged  as  to  give  the  rail  its  proper  shape. 
The  length  of  the  rail  was  usually  fifteen  feet,  the  bottom  of 

Fig.  228 — Represents  a  side  elevation  of  a  portion  of 
a  fish-beUied  rail. 

it  (Fig.  228)  presenting  an  undulating  outline  so  disposed  as 
to  give  the  rail  a  bearing  point  on  supports  placed  three  feet 
apart  between  their  centres.  This  form,  known  as  the  fish- 
'belly  rail,  was  adopted  as  presenting  the  greatest  strength  for 


432 


CIVIL  ENGINEERING. 


the  same  amount  of  metal.  It  has  been  found  on  trial  to  be 
liable  to  many  inconveniences.  The  rails  break  at  about 
nine  inches  from  the  supports,  or  one  fourth  of  the  distance 
between  the  bearing  points,  and  from  the  curved  form  of  the 
bottom  of  the  rail  they  do  not  admit  of  being  supported 
throughout  their  length. 

735.  The  form  of  rail  at  present  in  most  general  use  is 
known  by  the  name  of  the  j)arallel,  or  straiglit  rail,  the  top 
and  bottom  of  the  rail  being  parallel ;  or  as  the  T,  or  H  rail, 
from  the  form  of  the  cross-section. 

A  variety  of  forms  of  cross-section  are  to  be  met  with  in 
the  parallel  rail.    The  more  usual  form  is  that  (Fig.  229)  in 

Fig.  229 — ^Represents  a  cross-section  a 
of  a  parallel  rail  of  the  form  generally 
adopted  in  the  U.  States.  The  rail 
may  be  confined  to  its  chair  6  by 
two  wooden  keys  c  on  each  side, 
which  are  formed  of  hard  compressed 
■wood.  At  the  present  time  two  iron 
straps  are  used  instead  of  the-  keys  c 
c,  which  are  firaily  bolted  to  the  rails. 
This  form  is  called  a  fish- joint.  In 
this  case  the  projection  h  is  omitted. 
A  very  great  variety  of  splices  are  in 
use. 

which  the  top  is  shaped  like  the  same  part  in  the  fish-belly 
rail,  the  bottom  being  widened  out  to  give  the  rail  a  more 
stable  seat  on  its  supports.  In  some  cases  the  top  and  bot- 
tom are  made  alike  to  admit  of  turning  the  rail.  The  great- 
est deviation  from  the  usual  form  is  in  the  rail  of  the  Great 
Western  Railway  in  England  (Fig.  230),  and  the  Grand 
,Trunk  in  Canada  ;  but  this  form  is  rapidly  going  out  of  use. 

Fig.  230 — Represents  a  cross-section  of  the  rail  of  the  Great 
Western  Railway  in  England.  This  rail  is  laid  on  a  continu- 
ous support,  and  is  fastened  to  it  by  screws  on  each  side  of 
the  rail.  A  piece  of  tarred  felt  wa&  inserted  between  the 
base  of  the  rail  and  its  support. 

The  dimensions  of  the  cross-section  of  a  rail  should  be  such 
that  the  deflection  in  the  centre  between  any  two  points  of 
support,  caused  by  the  heaviest  loads  upon  the  track,  should 
not  be  so  great  as  to  cause  any  very  appreciable  increase  of 
resistance  to  the  force  of  traction.  The  greatest  deflection, 
as  laid  down  by  some  writers,  should  not  exceed  three-hun- 
dredths  of  an  inch  for  the  usual  bearing  of  three  feet  between 
the  points  of  support.  The  top  of  the  rail  is  usually  about 
two  and  a  half  inches  broad,  and  an  inch  in  depth.  This  has 
been  found  to  present  a  good  bearing  surface  tor  the  wheels, 
and  sufiicient  strength  to  prevent  the  top  from  being  crushed 


RAILWAYS. 


483 


by  the  weight  upon  the  rail.  The  thickness  of  the  rib  varies 
between  half  an  inch  to  three-fourths  of  an  inch ;  and  the 
total  depth  of  the  rail  from  three  to  five  inches.  The  thick- 
ness and  breadth  of  the  bottom  have  been  varied  according  to 
the  strength  and  stability  demanded  by  the  traffic. 

736.  Steel  Rails.  Rails  made  entirely  of  steely  or  of 
wrought  iron,  with  a  thin  bar  of  steel  forming  the  top  surface, 
or  steel-top^  or  steel-headed  rails  as  they  are  termed,  from  their 
superior  strength  and  durability,  are  coming  into  general  use 
in  replacing  the  worn-out  wrought-iron  rails  of  old  roads. 
Steel  obtained  from  any  of  the  usual  processes,  either  cast, 
puddled,  or  Bessemer  steel,  may  be  used  for  the  steel  heads 
of  rails. 

From  the  experience  of  Swedish  engineers  it  appears  that 
solid  Bessemer  steel  rails  of  the  best  charcoal  pig-iron  may  be 
made  10  per  cent,  lighter  than  the  best  English  wrought-iron 
rails,  a  result  which  has  been  carried  into  practice  on  the 
Austrian  railways. 

The  durability  of  iron  rails  appears  to  depend  principally 
upon  the  perfection  of  the  welding,  the  chief  cause  of  their 
want  of  dura])ility  arising  from  the  lamination  caused  by  im- 
perfect welding. 

Formerly  wrought-iron  rails  were  made  partly  by  hammer- 
ing and  partly  by  rolling.  At  present  rolling  alone  is  used,, 
and  the  results  are  said  to  be  more  satisfactory,  whilst  the  pro- 
cess of  manufacture  is  more  simple. 

The  resistance  to  wear  of  rails,  from  English  experience,  it 
is  said,  may  be  measured  by  the  product  of  the  speed  and  of 
the  weight  passing  over  them.  The  rule  proposed  for  the- 
work  that  rails  may  be  subjected  to  is  220,000,000  tons  trans- 
ported at  the  rate  of  one  mile  per  hour.  The  length  of 
time  that  iron  rails  will  last  in  any  given  case  will  be  found  by 
multiplying  the  number  of  tons  transported  by  the  rate  of 
speed  per  hour  and  dividing  by  220. 

737.  Supports.  The  rails  are  laid  upon-,  supports  of  tim- 
ber or  stone.  The  supports  should  present  a  firm,  unyield- 
ing bed  to  the  rails,  so  as  to  prevent  all  displacement,  either 
in  a  lateral  or  a  vertical  direction,  from  the  pressure  thrown 
upon  them. 

Considerable  diversity  is  to  be  met  with  in  the  practice  of 
engineers  on  this  point.  On  the  earlier  roads,  heavy  stone 
blocks  were  mostly  used  for  supports,  but  these  were  found  to 
require  great  precautions  to  render  them  firm,  and  they  were,, 
moreover,  liable  to  split  from  the  means  taken  to  confine  the^ 
rails  to  them.  Timber  is  generally  preferred  to  stone.  It 
38 


434 


CIVIL  ENGINEERING. 


affords  a  more  agreeable  road  for  travel,  and  gives  a  better 
lateral  support  to  the  rails  than  stone  blocks,  and  the  wear 
upon  the  locomotive  and  other  machinery  is  less  severe. 

The  usual  method  of  placing  timber  supports  is  transversely 
to  the  track,  each  support,  termed  a  sleeper,  or  cross-tie, 
being  formed  of  a  piece  of  timber  six  or  eight  inches  square. 
The  ordinary  distance  between  the  centre  lines  of  the  sup- 
ports is  three  feet  for  rails  of  the  usual  dimensions.  With  a 
greater  bearing,  rails  of  the  ordinary  dimensions  do  not  pre- 
sent sufficient  stiifness.  The  sleepers,  when  formed  of  round 
timber,  should  be  squared  on  the  upper  and  lower  surface. 
On  some  of  the  recent  railways  in  England,  sleepers  present- 
ing in  cross  section  a  right-angled  triangle  have  been  used, 
the  right  angle  being  at  the  bottom.  They  are  represented  to 
be  more  convenient  in  setting,  and  to  offer  a  more  stable  sup- 
port than  those  of  the  usual  form.  The  sleepers  are  placed 
either  upon  the  ballasting  of  the  roadway,  or  upon  longitudi- 
nal beams  laid  beneath  them  along  the  line  of  the  rails.  The 
latter  is  indispensable  upon  new  embankments  to  prevent  the 
ends  of  the  sleepers  from  settling  unequally.  Thick  plank, 
about  eight  inches  broad  and  three  or  four  inches  thick,  is 
usually  employed  for  the  longitudinal  supports  of  the  sleepers. 

On  some  of  the  more  recent  railways  in  England,  the  rails 
have  been  laid  upon  longitudinal  beams,  presenting  a  con- 
tinuous support  to  the  rail,  the  beams  resting  upon  cross-ties. 

738.  Ballast.  A  covering  of  broken  stone,  of  clean  coarse 
gravel,  or  of  any  other  material  that  will  allow  the  water  to 
drain  off  freely,  is  laid  upon  the  natural  surface  of  the  excava- 
tions and  embankments,  to  form  a  firm  foundation  for  the 
supports.  This  has  received  the  appellation  of  the  hallast. 
Its  thickness  is  from  nine  to  eighteen  inches.  Open  or  broken- 
stone  drains  should  be  placed  beneath  the  ballasting  to  convey 
off  the  surface  water.  The  parts  of  the  ballasting  upon  which 
the  supports  rest  should  be  well  rammed,  or  rolled ;  and  it 
should  be  well  packed  beneath  and  around  the  supports. 
After  the  rails  are  laid,  another  layer  of  broken  stone  or 
gravel  should  be  added,  the  surface  of  which  should  be 
slightly  convex  and  about  three  inches  below  the  top  of  the 
rails. 

739.  Temporary  Railways  of  Wood  and  Iron.    On  the 

first  introduction  of  railways  into  the  United  States,  the  tracks 
were  formed  of  flat  iron  bars  laid  upon  longitudinal  beams. 
The  iron  bars  were  about  two  and  a  half  inches  in  breadth, 
and  from  one-half  to  tliree-fourtlis  of  an  inch  in  thickness,  the 
top  surface  being  slightly  convex.    They  were  placed  on  the 


RAILWAYS. 


435 


longitudinal  beams,  a  little  back  from  the  inner  edge,  the 
side  of  the  beam  near  the  top  being  bevelled  off,  and  were 
fastened  to  the  beam  by  screws  or  spikes,  which  passed 
through  elliptical  holes  with  a  countersink  to  receive  the 
lieads'  of  the  spikes ;  the  holes  receiving  this  shape  to  allow 
of  the  contraction  and  expansion  of  tlie  bar,  without  displac- 
ing the  fastenings.  The  longitudinal  beams  were  supported 
by  cross  sleepers,  with  which  they  were  connected  by  wedges 
that  confined  the  beams  in  notches  cut  into  the  sleepers  to  re- 
ceive them.  The  longitudinal  beams  were  usually  about  six 
inches  in  breadth,  and  nine  inches  in  depth,  and  in  as  long 
lengths  as  they  could  be  procured.  The  joints  between  the 
bars  were  either  square  or  oblique,  and  a  piece  of  iron  or  zinc 
was  inserted  into  the  beams  at  the  joint,  to  prevent  the  end 
of  the  rail  from  being  crushed  into  the  wood  by  the  wheels. 

In  some  instances  the  bars  were  fastened  to  long  stone 
blocks,  but  this  method  was  soon  abandoned,  as  the  stone  was 
rapidly  destroyed  by  the  action  of  the  wheels  ;  besides  which, 
the  rigid  nature  of  the  stone  rendered  the  travelling  upon  it 
excessively  disagreeable. 

This  system  of  railway,  whose  chief  recomlnendation  is 
economy  in  the  first  cost,  has  gradually  given  place  to  the 
solid  rail.  Besides  the  want  of  durability  of  the  structure,  it 
does  not  possess  sufiicient  strength  for  a  heavy  traffic. 

740.  Gauge.  The  distance  between  the  two  lines  of  rails 
of  a  track,  termed  the  gauge,  wliich  has  been  adopted  for  the 
great  majority  of  the  railways  in  England,  and  also  with  us, 
is  4  feet  8|-  inches.  This  gauge  appears  to  have  been  the  re- 
sult of  chance,  and  it  lias  been  followed  in  the  great  majority 
of  cases  up  to  the  present  time,  owing  to  the  inconvenience 
that  would  arise  from  the  adoption  of  a  different  gauge  upon 
new  lines.  The  greatest  deviation  yet  made  from  the  estab- 
lished gauge  is  in  that  of  the  Grreat  Western  Railway,  in 
which  the  gauge  is  seven  feet,  Engineers  are  generally 
agreed  that  with  a  wider  gauge  the  wheels  of  railway  cars 
could  be  made  of  greater  diameter  than  they  now  receive, 
and  be  placed  outside  of  the  cars  instead  of  under  them  as  at 
present ;  the  centre  of  gravity  of  the  load  might  be  placed 
lower,  and  more  steadiness  of  motion  and  greater  security  at 
high  velocities  be  attained.  All  roads  having  a  gauge  above 
4  ieet  8  J  inches  are  inclined  rather  to  reduce  them  to  that 
gauge  or  use  a  third  rail  so  as  to  run  the  cars  of  that  gauge 
over  their  own. 

"Within  the  last  four  or  five  years  the  subject  of  roads  of 
very  na/rrow  gauge  has  been  much  discussed.    The  advan- 


436 


CIVIL  ENGINEERING. 


tages  principally  claimed  for  roads  of  this  kind  are:  1st, 
great  reduction  in  first  cost ;  2d,  allowing  steeper  grades  and 
curves  of  smaller  radius  ;  3d,  less  wear  and  tear  on  the  road 
on  accoimt  of  the  rolling  stock  being  much  lighter ;  4th,  the 
ratio  of  live  to  dead  weight  is  much  less.  Some  lines  have 
been  made  with  a  2^-foot  gauge,  but  the  advocates  of  narrow 
gauge  generally  recommend  a  3-foot  gauge.  The  latter  is 
the  gauge  of  the  Denver  and  Texas  narrow-gauge  road. 

In  a  double  track  the  distance  between  the  two  tracks  is 
generally  the  same  as  the  gauge  ;  and  the  distance  between 
the  outside  rail  of  a  track,  and  the  sides  of  the  excavation, 
or  embankment,  is  seldom  made  greater  than  six  feet,  as  this 
is  deemed  sufficient  to  prevent  the  cars  from  going  over  an 
embankment  were  they  to  run  off  the  rails. 

741.  On  all  straight  portions  of  a  track,  the  supports  should 
be  on  a  level  transversely,  and  parallel  to  the  plane  of  the 
track  longitudinally.  The  top  surface  of  the  rail  should  in- 
cline inward,  to  conform  to  the  conical  form  of  the  wheels ; 
this  is  now  usually  effected  by  giving  the  chair  the  requisite 
pitch,  or  by  forming  the  top  surface  with  the  requisite  bevel 
for  this  purpose. 

742.  Curves.  In  the  curved  portions  of  a  track  the  cen- 
trifugal force  tends  to  force  the  carriage  towards  the  outside 
rail  of  the  curve,  and  by  elevating  the  outer  rail  the  force  of 
gravity  tends  to  draw  it  towards  the  inside  rail.  From  the 
above  conditions  of  equilibrium  the  elevation  which  the  ex- 
terior rail  should  receive  above  the  interior  can  be  readily 
calculated.  The  method  adopted  is  to  give  the  exterior  rail 
an  elevation  sufficient  to  prevent  the  flanch  of  the  wheel  from 
being  driven  against  the  side  of  the  rail  when  the  car  is  mov- 
ing at  the  highest  supposed  velocity ;  or,  in  other  words,  to 
give  the  inclined  plane  across  the  track,  on  which  the  wheels 
rest,  an  inclination  such  that  the  tendency  of  the  w^heels  to 
slide  towards  the  interior  rail  shall  alone  counteract  the  cen- 
trifugal force. 

743.  Sidings,  etc.  On  single  lines  of  railways  short  por- 
tions of  a  track,  termed  sidings,  are  placed  at  convenient  in- 
tervals along  the  main  track,  to  enable  cars  going  in  opposite 
directions  to  cross  each  other,  one  train  passing  into  the  siding 
and  stopping  while  the  other  proceeds  on  the  main  track. 
On  double  lines  arrangements,  termed  crossings,  are  made  to 
enable  trains  to  pass  from  one  track  into  the  other,  as  circum- 
stances may  require.  The  position  of  sidings  and  their 
length  will  depend  entirely  on  local  circumstances,  as  the 
length  of  the  trains,  the  number  daily,  etc. 


RAILWAYS. 


437 


The  manner  generally  adopted,  of  connecting  the  main 
track  with  a  siding,  or  a  crossing,  is  very  simple.  It  consists 
(Fig.  231)  in  having  two  short  lengths  of  the  opposite  rails 

Fig.  231  —  Represents 
the  sliding  switches, 
or  rails,  for  connect- 
ing a  siding  with  the 
1^      main  track. 

a,  o,  rails  connected 
by  an  iron  rod  6,  by 
which  they  can  be 
turned   around  the 
(L       joints  o,  o. 

c,  c,   rails   of  main 
track. 

d,  d,  rails  of  siding. 

of  the  main  track,  where  the  siding  or  crossing  joins  it, 
movable  around  one  of  their  ends,  so  that  the  other  can  be 
displaced  from  the  line  of  the  main  track,  and  be  joined 
with  that  of  the  siding,  or  crossing,  on  the  passage  of  a  car 
out  of  the  main  track.  These  movable  portions  of  rails  are 
connected  and  kept  parallel  by  a  long  cross-bolt,  to  the  end 


Fig.  2.32 — Represents  a  plain  M,  and  section  N,  of  a  fixed  crossing  plate.  The  plate  A  is  of 
casi>iron,  with  vertical  ribs  c,  c,  on  the  bottom,  to  give  it  the  requisite  strength.  Wrought- 
iron  bars  a,  a,  placed  in  the  lines  of  the  two  intersecting  rails  rf,  are  firmly  screwed  to 
the  plate ;  a  sufficient  space  being  left  between  them  and  the  rails  for  the  flanch  of  the 
wheel  to  pass. 


of  which  a  vertical  lever  is  attached  to  draw  them  forward,  or 
shove  them  back. 

At  the  point  where  the  rails  of  the  two  tracks  intersect,  a 
cast-iron  plate,  termed  a  Grossing-2)late  (Fig.  232),  is  placed  to 
connect  the  rails.  The  surface  of  the  plate  is  arranged  either 
with  grooves  in  the  lines  of  the  rails  to  admit  the  flanch  of 
the  wheel  in  passing,  the  tire  running  upon  the  surface  of  the 
plate ;  or  wrought-iron  bars  are  aflixed  to  the  surface  of  the 
plate  for  the  same  purpose. 

The  angle  between  the  rails  of  the  main  tracks  and  those 
of  a  siding  or  crossing,  termed  the  angle  of  deflection,  should 
not  be  greater  than  2^  or  3^.    The  connecting  rails  between 


438 


CIVIL  ENGINEERING. 


the  straight  portions  of  tlie  tracks  should  be  of  the  shape  of 
an  S  curve,  in  order  that  the  passage  may  be  gradually 
effected.  At  the  present  time  switch  rails  and  frogs  of  pecu- 
liar construction  are  in  use,  which  are  so  made  and  arranged 
as  to  leave  the  main  track  unbroken,  so  that  if  the  switch  is 
wrongly  placed  the  train  on  the  main  track  will  not  run  oJff. 
There  are  many  devices  for  securing  this  result. 

744.  Turn-plates.  Where  one  track  intersects  another 
under  a  considerable  angle,  it  will  be  necessary  to  substitute 
for  the  ordinary  method  of  connecting  them,  what  is  termed 
a  turn-jplate,  or  turn-table.  This  consists  of  a  strong  circular 
platform  of  wood  or  cast  iron,  movable  around  its  centre  by 
means  of  conical  rollers  beneath  it  running  upon  iron  roller- 
ways.  Two  rails  are  laid  upon  the  platform  to  receive  the 
car,  which  is  transferred  from  one  track  to  the  other  by  turn- 
ing the  platform  sufficiently  to  place  the  rails  upon  it  in  the 
same  line  as  those  of  the  track  to  be  passed  into. 

745.  Street  crossings.  When  a  track  intersects  a  road,  or 
street,  upon  the  same  level  with  it,  the  rail  must  be  guarded 
by  cast-iron  plates  laid  on  each  side  of  it,  sufficient  space  be- 
ing left  between  them  and  the  rail  for  the  play  of  the  fianch. 
The  top  of  the  plates  should  be  on  a  level  with  the  top  of  the 
rail.  Wherever  it  is  practicable  a  drain  should  be  placed  be- 
neath, to  receive  the  mud  and  dust  which,  accumulating  be- 
tween the  plates  and  rail,  might  interfere  with  the  passing  of 
the  cars  along  the  rails. 

746.  Gradients.  From  various  experiments  upon  the 
friction  of  cars  upon  railways,  it  appears  that  the  angle  of 
repose  is  about  -g-J-Q,  but  that  in  descending  gradients  much 
steeper,  the  velocity  due  to  the  accelei-ating  force  of  gravity 
soon  attains  its  greatest  limit  and  remains  constant,  from  the 
resistance  caused  by  the  air. 

The  limit  of  the  velocity  thus  attained  upon  gradients  of 
any  degree,  whether  the  train  descends  by  the  action  of  grav- 
ity alone,  or  by  the  combined  action  of  the  motive-power  of 
the  engine  and  gravity,  can  be  readily  determined  for  any 
given  load.  From  calculation  and  experiment  it  appears  that 
heavy  trains  may  descend  gradients  of  jlo"?  without  attaining 
a  greater  velocity  than  about  40  or  50  miles  an  hour,  by  al- 
lowing them  to  run  freely  without  applying  the  brake  to 
check  the  speed.  Ey  the  application  of  the  brake,  the  velo- 
city may  be  kept  within  any  limit  of  safety  upon  much  steeper 
gradients.  The  only  question,  then,  in  comparing  the  ad- 
vantages of  different  gradients,  is  one  of  the  comparative  cost 
between  the  loss  of  power  and  speed,  on  the  one  hand,  for 


RAILWAYS. 


439 


ascending  trains  on  steep  gradients,  and  that  of  the  heavy  ex- 
cavations, tunnels,  and  embankments  on  the  other,  which 
may  be  required  by  lighter  gradients. 

In  distril)uting  tlie  gradients  along  a  line,  engineers  are 
generally  agreed  that  it  is  more  advantageous  to  have  steep 
gradients  upon  short  portions  of  the  line,  than  to  overcome 
the  same  difference  of  level  by  gradients  less  steep  upon 
longer  developments. 

747.  In  steep  gradients ^  where  locomotive  power  cannot  be 
employed,  stationary  power  is  used,  the  trains  being  dragged 
up,  or  lowered,  by  ropes  connected  with  a  suitable  mechan- 
ism, worked  by  stationary  power  placed  at  the  top  of  the 
plane.  The  inclined  planes,  with  stationary  powers,  gener- 
ally receive  a  uniform  slope  throughout.  The  portion  of  the 
track  at  the  top  and  bottom  of  the  ^^lane  should  be  level  for 
a  sufficient  distance  back,  to  receive  the  ascending  or  descend- 
ing trains.  The  axes  of  the  level  portion  should,  when  prac- 
ticable, be  in  the  same  vertical  plane  as  that  of  the  axis  of  the 
inclined  plane. 

Small  rollers,  or  sheeves,  are  placed  at  suitable  distances 
along  the  axis  of  the  inclined  plane,  upon  which  the  rope 
rests. 

Within  a  few  years  back  flexible  bands  of  rolled  hoop-iron 
have  been  substituted  for  ropes  on  some  of  the  inclined 
planes  of  the  United  States,  and  have  been  found  to  work 
well,  presenting  more  durability  and  being  less  expensive 
than  ropes. 

On  very  steep  gradients  the  expedient  of  a  third  rail 
in  the  centre  of  the  track,  and  raised  rather  above  the  plane 
of  the  other  two  rails,  has  been  used.  Two  horizontal  wheels 
underneath  the  locomotive  run  on  this  rail,  and  may  be 
tightened  to  any  desirable  degree  of  compression  on  it.  In 
this  way  a  gradient  of  440  feet  per  mile  is  used  over  Mont 
Cenis.  Without  the  intermediate  rail  grades  as  steep  as  280, 
and  in  one  case  304  feet  per  mile,  have  been  ascended  by 
means  of  the  adhesive  power  of  the  locomotive  only.  But 
such  grades  will  never  be  sought;  on  the  other  hand, 
they  will  be  avoided  when  possible.  Grades  of  50  and  60 
feet  to  the  mile  are  very  common.  The  maximum  grade 
allowable  by  law  on  the  Central  Pacific  Kailroad  is  the 
same  as  that  of  the  Baltimore  and  Ohio  Railroad,  viz.,  116 
feet  per  mile. 

748.  Tunnels.  The  choice  between  deep  cutting  and  tun- 
nelling, will  depend  upon  the  relative  cost  of  the  two,  and  the 
nature  of  the  ground.    When  the  cost  of  the  two  methods 


440 


CIVIL  ENGINEEEING. 


would  be  about  equal,  and  the  slopes  of  tlie  deep  cut  are  not 
liable  to  slips,  it  is  usually  more  advantageous  to  resort  to 
deep  cutting  than  to  tunnelling.  So  much,  however,  will  de- 
pend upon  local  circumstances,  that  the  comparative  advan- 
tages of  the  two  methods  can  only  be  decided  upon  under- 
standingly  when  these  are  known. 

749.  The  operations  in  Tunnelling  will  depend  upon  the 
nature  of  the  soil.  The  woi'k  is  commenced  by  setting  out,  in 
the  first  place,  with  great  accuracy  uj)on  the  surface  of  the 
ground,  the  profile  line  contained  in  the  vertical  plane  of  the 
axis  of  the  tunnel.  At  suitable  intervals  along  this  line 
vertical  pits,  termed  worhing  shafts^  are  sunk  to  a  level  with 
the  top,  or  crowm  of  the  tunnel.  The  shafts  and  excavations, 
which  form  the  entrances  to  the  tunnel,  are  connected,  when 
the  soil  will  admit  of  it,  by  a  small  excavation  termed  a 
heading,  or  drift,  usually  five  or  six  feet  in  width,  and  seven 
or  eight  feet  in  height,  w^iic-h  is  made  along  the  crown  of 
the  tunnel.  After  the  drift  is  completed,  the  excavation  for 
the  tunnel  is  gradually  enlarged ;  the  excavated  earth  is 
raised  through  the  working  shafts,  and  at  the  same  time 
carried  out  at  the  ends.  The  dimensions  and  form  of  the  cross 
section  of  the  excavation  will  depend  upon  the  nature  of 
the  soil  and  the  object  of  the  tunnel  as  a  communication. 
In  solid  rock  the  sides  of  the  excavation  are  usually  vertical ; 
the  top  receives  an  arched  form  ;  and  the  bottom  is  horizontal. 
In  soils  which  require  to  be  sustained  by  an  arch,  the  excava- 
tion should  conform  as  nearly  as  practicable  to  the  form  of 
cross  section  of  the  arch. 

In  tunnels  through  unstratified  rocks,  the  sides  and  roof 
may  be  safely  left  unsupported ;  but  in  stratified  rocks  there 
is  danger  of  blocks  becoming  detached  and  falling  ;  wherever 
this  is  to  be  apprehended,  the  top  of  the  tunnel  should  be 
supported  by  an  arch. 

Tunnelling  in  loose  soils  is  one  of  the  most  hazardous 
operations  of  the  miner's  art,  requiring  the  greatest  precau- 
tions in  supporting  the  sides  of  the  excavations  by  strong 
rough  framework,  covered  by  a  sheathing  of  boards,  to  secure 
the  workmen  from  danger.  When  in  such  cases  the  drift 
cannot  be  extended  throughout  the  line  of  the  tunnel,  the 
excavation  is  advanced  only  a  few  feet  in  each  direction 
from  the  bottom  of  the  working  shafts,  and  is  gradually 
widened  and  depended  to  the  proper  form  and  dimensions  to 
receive  the  masonry  of  the  tunnel,  which  is  immediately 
commenced  below  each  working  shaft,  and  is  carried  forward 
in  both  directions  towards  the  two  ends  of  the  tuimel. 


RAILWAYS.  441 

750.  Masonry  of  Tunnels.  The  cross  section  of  the  arch 
of  a  tunnel  (Fig.  233)  is  usually  an  oval  segment,  formed  of 


Fig.  233 — Represents  the  general  form  of 
the  cross  section  o  of  a  brick  arch  for 
tunnels. 

a,  a,  askew-back  stone  between  the  sides 
of  the  arch  and  the  bottom  inverted 
arch. 


arcs  of  circles  for  the  sides  and  top,  resting  on  an  inverted 
arch  at  bottom.  The  tunnels  on  some  of  the  recent  railways 
in  England  are  from  24  to  30  feet  wide,  and  of  the  same 
height  from  the  level  of  the  rails  to  the  crown  of  the  arch. 
The  usual  thickness  of  the  arch  is  eighteen  inches.  Brick 
laid  in  hydraulic  cement  is  generally  used  for  the  masonry, 
an  askew-back  course  of  stone  being  placed  at  the  junction  of 
the  sides  and  the  inverted  arch.  The  masonry  is  constructed 
in  short  lengths  of  about  twenty  feet,  depending,  however, 
upon  the  precautions  necessary  to  secure  the  sides  of  the  ex- 
cavation. As  the  sides  of  the  arch  are  carried  up,  the  frame- 
work supporting  the  earth  behind  is  gradually  removed,  and 
the  space  between  the  back  of  the  masonry  and  the  sides  of 
the  excavation  is  filled  in  with  earth  well  rammed.  This 
operation  should  be  carefully  attended  to  throughout  the 
whole  of  the  backing  of  the  arch,  so  that  the  masonry  may 
not  be  exposed  to  the  effects  of  any  sudden  yielding  of  the 
earth  around  it. 

751.  The  earth  at  the  ends  of  the  tunnel  is  supported  by  a 
retaining  wall,  usually  faced  with  stone.  These  walls,  termed 
the  fronts  of  the  tunnel,  are  generally  finished  with  the 
■usual  architectural  designs  for  gatew^ays.  To  secure  the  ends 
of  the  arch  from  the  pressure  of  the  earth  above  them,  cast- 
iron  plates  of  the  same  shape  and  depth  as  the  top  of  the 
arch,  are  inserted  within  the  masonry,  a  short  distance  from 


442 


CIVIL  ENGINEERING. 


the  ends,  and  are  secured  by  wronglit-iron  rods  firmly 
anchored  to  the  masonry  at  some  distance  from  each  end. 

752.  The  working  shafts,  which  are  generally  made  cylin- 
drical and  faced  with  brick,  rest  nj^on  strong  curbs  of  cast 
iron,  inserted  into  the  masonry  of  the  arch.  The  diameter  C)f 
the  shaft  within  is  ordinarily  nine  feet. 

753.  The  ordinary  difficulties  of  tunnelling  are  greatly  in- 
creased by  the  presence  of  water  in  the  soil  through  whicli 
the  work  is  driven.  Pumps,  or  other  suitable  machinery  for 
raising  water,  placed  in  the  working  shafts,  will  in  some 
cases  be  requisite  to  keep  them  and  the  drift  free  from  water 
until  an  outlet  can  be  obtained  for  it  at  the  ends,  by  a  drain 
along  the  bottom  of  the  drift.  Sometimes,  when  the  water  is 
found  to  gain  upon  the  pumps  at  some  distance  above  the 
level  of  the  crown  of  the  tunnel,  an  outlet  may  be  obtained 
for  it  by  driving  above  the  tunnel  a  drift-way  between  the 
shafts,  giving  it  a  suitable  slope  from  the  centre  to  tlie  two 
extremities  to  convey  the  water  off  rapidly. 

In  tunnels  for  railways,  a  drain  should  be  laid  under  the 
balasting  along  the  axis,  upon  the  inverted  arch  of  the  bottom. 

Tunnelling  in  rock  is  greatly  facilitated  at  the  present 
day  by  power-drilling-machines,  which  are  driven  by  com- 
pressed air.  By  this  means  they  are  able  to  advance  three 
times  as  fast  as  by  hand  labor.  The  compressed  air  greatly 
facilitates  ventilation.  The  Mont  Cenis  tunnel  (nearly  T  miles 
long)  and  the  Hoosac  tunnel  (about  4  miles  long)  have  been 
driven  in  this  way,  and  the  St.  Godard  tunnel  (nearly  13  miles 
long)  is  BOW  in  process  of  construction  on  the  same  plan. 

754.  The  following  extracts  are  made  from  a  series  of 
papers,  published  in  the  London  Engineering^  from  Oct.  7, 
1S70,  to  December  30,  1870,  giving  a  translation  of  a  work 
by  Baron  von  Weber,  Director  of  the  State  Railways  of  Sax- 
ony, T\dth  running  comments  by  the  translator,  detailing  the 
experiments  made  by  the  author,  and  giving  his  deductions 
from  them,  on  the  Stability  of  the  Permanent  Way. 

Baron  von  Weber  desired,  in  the  first  place,  to  ascertain 
what  was  the  minimum  thickness  which  would  be  given  to 
the  web  of  a  rail,  in  order  that  the  latter  might  still  possess 
a  greater  power  of  resistance  to  lateral  forces  than  the  fasten- 
ings by  which  it  was  secured  to  the  sleepers. 

755.  Resistance  of  Rail  to  Ijateral  Forces.  Prom  the 
experiments  the  result  was  deduced,  that  the  least  thickness 
ever  given  to  the  webs  of  rails  in  practice  is  more  than  suf- 
ficient, and  that  if  it  were  possible  to  roll  webs  \  in.  thick, 
such  webs  would  be  amply  strong,  if  it  were  not  that  there 


RAILWAYS. 


443 


would  be  a  chance  of  their  being  torn  at  the  points  where 
they  are  traversed  by  the  fish-plate  bolts.  Baron  von  Weber 
concludes  that  webs  f  in.  or  ^  in.  thick  are  amply  strong 
enough  for  rails  of  any  ordinary  height,  and  that  in  fact  the 
webs  should  be  made  as  thin  as  the  pi-ocess  of  rolling,  and 
as  the  provision  of  sufficient  bearing  for  the  fish-plate  bolts 
will  permit. 

756.  Stability  of  the  Permanent  Way.  The  stability  of 
a  permanent  way  structure  in  a  longitudinal  direction,  is  con- 
sidered by  Baron  von  Weber  as  depending  upon  the  bed- 
ding of  the  sleepers  in  the  ballast,  the  friction  of  the  rails  up- 
on the  sleepers,  the  strength  of  the  spikes  or  other  fastenings, 
and,  lastly,  upon  the  strength  of  the  connections  between  the 
ends  of  the  rails.  These  connections  have,  in  the  fii'St  place, 
to  keep  the  heads  of  the  rails  in  their  proper  position  with  re- 
gard to  each  other  ;  next,  to  give  to  the  joint  a  certain  amount 
of  rigidity ;  and  finally,  to  insure  that  the  horizontal  or  verti- 
cal deflections  of  the  two  rails  connected  take  place  together. 
Of  the  many  forms  of  connections  which  have  from  time  to 
time  been  proposed  for  rails,  but  two  practically  fulfil  the  con- 
ditions just  mentioned,  these  two  being  the  joint  chair  and 
the  fish-joint,  in  their  various  modifications  and  forms. 

We  now  come  to  the  researches  made  by  Baron  von  Weber 
to  determine  the  power  of  permanent  way  structures  to  resist 
forces  tending  to  displace  the  entire  system.  Baron  von 
Weber  states  that  as  the  speed  of  trains  was  increased  on  Ger- 
man railways,  there  was  noticed  a  peculiar  and  dangerous 
displacement  of  the  permanent  way,  this  displacement  taking 
place  chiefiy  where  trains  pass  from  straight  to  curved  por- 
tions of  the  line,  or  from  curved  portions  to  level  and  straight 
lengths,  over  which  they  passed  at  an  increased  speed.  It  was 
also  observed  that  the  displacements  at  the  first-mentioned 
points — displacements  which  consisted  in  the  shifting  of  the 
line  towards  the  convex  side  of  the  curves — were  caused  prin- 
cipally by  engines  having  long  wheel  bases  and  a  compara- 
tively light  load  on  the  leading  wheels  ;  while  the  displace- 
ment of  the  straight  portions  of  the  lines  was  due  mainly  to 
the  action  of  powerful  engines  with  short  wheel  bases  and 
considerable  overhang  on  each  end.  In  this  latter  case  the 
horizontal  oscillations  which  produced  the  displacements  were 
almost  always  found  to  arise  from  the  effect  of  vertical  im- 
jDact  due  to  a  loose  joint  or  some  local  settlement  in  the  line, 
the  engine  being  thus  not  merely  caused  to  lurch  heavily  side- 
ways, but  being  also  made  to  oscillate  in  a  vertical  plane,  thus 
alternately  relieving  and  increasing  the  loads  on  the  leading 


444 


CrVTL  ENGINEERING. 


and  trailing  wheels.  Under  these  circumstances,  when  the 
flange  of  the  leading  wheel  struck  the  rail  laterally  at  the 
same  time  that  the  load  on  the  latter  was  decreased  by  the 
momentary  relief  of  the  leading  wheel  from  a  portion  of  the 
weight  it  ought  to  carry,  there  was  a  greater  displacement 
than  there  otherwise  would  have  been  owing  to  the  dimin- 
ished friction  between  the  permanent  way  structure  and  its 
foundation.  Both  the  classes  of  displacements  to  which  we 
have  referred  were  found  to  be  less  in  permanent  way  struc- 
tures possessing  considerable  vertical  rigidity  than  in  those 
of  a  more  flexible  character. 

757.  Experiments  on  the  Power  of  Permanent  Way- 
structures  to  resist  Horizontal  Displacements  of  the 
entire  System.  These  experiments  were  made  to  obtain 
answers  to  the  five  following  questions : — 

a.  What  is  the  resistance  offered  by  a  well-bedded  sleeper 
of  average  size  against  lateral  displacement  in  the  ballast  ? 

h.  Wliat  is  the  resistance  of  the  whole  structure  against 
displacement  at  one  point,  and  what  is  the  influence  of  the 
ballast  and  bedding,  on  and  in  which  the  structure  rests,  upon 
this  resistance  % 

G.  How  far  does  the  filling  against  the  ends  of  the  sleepers 
increase  this  resistance  \ 

d.  To  what  extent  is  the  resistance  to  lateral  displacement 
increased  by  the  load  on  the  structure  ? 

e.  How  far  does  the  application  of  piles  or  stones,  etc., 
etc.,  increase  this  resistance  ? 

The  deductions  to  be  made  from  the  experiments  referring 
to  questions  a  and  5,  Baron  von  Weber  considers  to  be 
as  follows :  1st.  The  resistance  of  unloaded  well-bedded  per- 
manent way-structures  is  comparatively  small,  a  lateral 
pressure  of  from  30  to  50  centners  being  suflicient  to  break 
the  connection  between  the  sleeper  and  the  ground.  This 
pressure  is  less  than  that  which  would  be  exerted  by  the 
centrifugal  force  due  to  the  passage  of  a  25-ton  locomo- 
tive through  a  curve  of  1,000  feet  I'adius,  at  a  speed  of  30 
miles  per  hour,  supposing  thai  this  centrifugal  force  Avas  not 
counteracted  by  superelevation  of  the  exterior  rail.  2d.  The 
nature  of  the  ballast  in  which  the  sleepers  of  unloaded  per- 
manent way-stru(itures  are  bedded  has  no  important  influence 
on  the  resistance  to  lateral  displacement.  3d.  The  pressure 
requisite  for  producing  the  horizontal  displacement  of  an  un- 
loaded structure  increases  until  this  displacement  has  reached 
a  certain  amount,  generally  between  12  and  18  millimetres 
(from  0.472  in.  to  0.708  in.),  when  the  further  displacement 


RAILWAYS. 


445 


up  to  50  to  75  millimetres  (2  in.  to  3  in.)  is  produced  without 
any  considerable  augmentation  in  the  pressure,  until  finally 
a  considerable  tension  is  set  up  in  the  different  parts  of  the 
structure. 

Baron  von  Weber's  conclusions  from  the  experiments  re- 
ferring to  question  c  are  as  follows :  1st.  ^  That  the  filling 
of  ballast  against  the  ends  of  the  sleepers,  up  to  the  top 
surface  of  the  latter,  has  an  insignificant  infiuence  upon 
the  resisting  power  of  the  structure  to  lateral  displacement, 
particularly  if  the  structure  is  unloaded,  and  if  a  one-sided 
tilting  is  possible.  2d.  That  if  the  ballast  is  not  filled  against 
the  ends  of  the  sleepers,  the  elasticity  of  the  rails  will  bring 
back  the  structure  into  its  original  position,  on  the  removal 
of  the  pressure,  even  after  considerable  displacement,  as  in 
this  case  small  portions  of  ballast  cannot  fall  between  the 
end  of  the  shifted  sleeper  and  the  undisturbed  end  filling, 
as  is  the  case  when  the  practice  of  filling  up  against  the  ends 
is  followed. 

We  now  come  to  the  experiments  made  by  Baron  von 
Weber  to  obtain  an  answer  to  question  d.  It  was,  of  course, 
requisite,  in  order  that  a  proper  comparison  might  be  insti- 
tuted, that  these  experiments  should  be  conducted  under  cir- 
cumstances as  nearly  as  possible  identical  with  those  which 
existed  when  the  resistance  of  displacement  of  the  unloaded 
structure  was  investigated ;  and  in  selecting  portions  of  per- 
manent way  for  the  last-mentioned  experiments,  therefore, 
such  lengths  were  chosen  as  would  afford  space  for  the  experi- 
ments with  the  loaded  structure,  without  introducing  any 
variations  in  bedding,  firmness  of  the  ballast,  etc.,  etc. 

The  results  of  seven  sets  of  trials  show  that  the  resistance 
of  the  structure  to  lateral  displacement  was  increased  almost 
tenfold  by  the  load  of  twenty-seven  tons ;  and  that  lateral 
pressures  vrhich  produced  in  the  unloaded  structure  displace- 
ments entirely  inadmissible  in  practice,  did  not  affect  the 
loaded  structure  in  any  perceptible  degree.  The  portion  of 
the  unloaded  structure  shifted  by  the  press  in  the  above  ex- 
periments weighed  almost  exactly  2^  tons,  while  the  total  mass 
moved,  including  the  filling  against  the  ends  of  the  sleepers, 
weighed  3  tons ;  and  taking  this  into  consideration,  it  appeared 
as  if  the  resistance  to  displacement  varied  directly — as  indeed 
it  might  have  been  supposed  it  would  do — as  the  weight 
resting  on  the  ground. 

Baron  von  Weber s  conclusion  with  regard  to  this  subject 
is,  that  the  force  required  to  produce  the  lateral  displace- 
ment of  a  permanent  way-structure  is  directly  proportionate 


446 


CmL  ENGINEERING. 


to  the  weight  by  which  the  structure  is  pressed  upon  the 
ground. 

758.  Experiments  relating  to  Question  e.  In  con- 
sidering the  influence  of  piles  or  stakes  driven  into  the  ballast 
against  the  ends  of  the  sleepers  to  prevent  lateral  shifting  of 
the  latter,  Baron  von  Weber  remarks  that  the  resisting  power 
of  such  piles  has  been  very  differently  "  estimated  "  by  rail- 
way engineers,  but  that  as  far  as  he  is  aware  the  advantages 
or  disadvantages  attending  the  use  of  such  piles  has  never 
been  ascertained  by  experiment.  Many  elements  evidently 
exercise  an  influence  on  the  lateral  displacement  of  piles 
driven  vertically  into  the  ground,  and  experiments  made 
with  a  view  of  ascertaining  the  lateral  resistance  of  such 
piles  can  only  show  in  a  very  general  manner  how  far  the  ad- 
vantages derived  from  their  use  will  counterbalance  the 
extra  expense  they  involve.  The  results  obtained  by  experi- 
ment are  moreover  liable  to  great  variations.  Thus,  a  pile 
driven  deeply  into  solid,  loam}^  soil,  offers  in  dry  weather 
great  resistance  to  lateral  displacenient,  whereas  after  a 
shower  of  rain — not  strong  enough  to  soak  into  the  ground, 
but  capable  of  penetrating  the  narrow  crack  formed  between 
the  jyWe  and  surrounding  earth  by  the  vibrations  caused  by 
the  traffic — the  upper  end  of  the  pile  can  be  moved,  by  the 
application  of  a  comparatively  small  force,  to  an  extent  sufli- 
eient  to  render  it  useless  as  a  means  of  lateral  support  for  the 
sleeper.  Thus  Baron  von  Weber  has  found  that  piles  which, 
in  dry  weather,  require  a  force  of  from  15  to  20  cwt.  to  shift 
their  heads  laterally  through  a  distance  of  one  inch,  could  be 
moved  to  the  same  extent  by  the  force  of  about  5  cwt.  after 
a  shower  of  rain  lasting  barely  one  hour. 

The  elements  by  which  the  lateral  stability  of  such  piles  as 
those  we  are  now  considering  is  affected  are :  the  diameter, 
length,  and  section  of  the  pile,  the  description  of  wood  of 
which  it  is  made,  and  the  nature  of  ground  into  which  it  is 
driven.  To  determine  the  influence  of  all  these  elements  in 
their  various  combinations  a  very  extensive  series  of  experi- 
ments would  have  been  required,  and  Baron  von  Weber 
therefore  confined  his  researches  to  ascertaining  the  maxi- 
mum resistance  of  such  stakes  as  are  used  on  the  Saxon 
state  railways,  availing  himself,  however,  of  all  available 
opportunities  of  noticing  the  resistance  under  unfavorable 
circumstances. 

The  principle  was  laid  down  that  a  displacement  of  the  top 
of  a  pile  to  tlie  extent  of  10  millimetres  (==0.39  in.)  should 
be  considered  as  inconsistent  with  its  further  usefidness. 


EAILWAYS. 


In  this  series  of  trials  the  pressure  acted  against  a  number 
of  oak  stakes,  some  of  round  and  some  of  square  section,  and 
var}ang  from  2  ft.  11 J  in.  to  3  ft.  11 J  in.  long.  The  ground 
was  solid  sand  or  mixed  gravel,  and  some  of  the  stakes  had 
been  in  use  for  a  considerable  time,  while  others  were  driven 
expressly  for  the  experiments.  The  results  showed  that  a 
pressure  of  from  3  cwt.  to  5  cwt.  was  quite  sufficient  to  pro- 
duce the  lateral  displacement  of  10  millimetres  (=0.39  in.) 
whilst  a  pressure  of  7  cwt.  almost  forced  the  stakes  out  of  the 
ground.  These  experiments  showed,  therefore,  that  in 
ground  of  this  kiiid  piles  driven  against  the  ends  of  the 
sleepers  could  not  exercise  the  least  influence  upon  the 
stability  of  the  permanent  way-structure. 

In  these  trials  the  pressure  acted  against  a  pile  4  in.  in 
diameter  and  2  ft.  11^  in.  long,  driven  into  a  heavy  loamy 
ballast,  which  had  been  laid  down  about  ten  years  over  the 
broken-stone  bedding  of  an  old  line.  The  results  which  we 
subjoin  show  that  the  resisting  power  of  such  a  pile  would  be 
of  but  little  use  for  increasing  the  lateral  stability  of  the 
structure. 

Three  trials  were  made  on  a  pile  4  in.  square  and  4  ft. 
11  in.  long,  driven  into  the  same  ground  as  the  pile  tested  in 
the  last  series  of  experiments. 

The  results  showed  that  the  length  and  section  of  the  pile 
exercise  an  important  influence  on  its  resistance  to  lateral 
pressure.  It  was  found  in  these  last  two  series  of  experiments 
that  when  the  displacement  of  the  piles  became  great,  the 
ground  behind  them  cracked  radially  and  rose  considerably  ; 
while,  when  the  cracks  reached  certain  dimensions,  it  was 
found  that  no  increase  of  pressure  was  required  to  produce 
a  further  displacement  of  the  piles. 

Baron  von  Weber's  conclusions,  drawn  from  the  experi- 
ments relating  to  question  ^,  are  as  follows :  1st.  That  the 
resistance  of  piles  driven  into  sandy  or  other  light  ground  is 
so  insignificant  that  the  use  of  such  piles  under  such  circum- 
stances will  not  produce  an  increased  stability  of  the  structure 
against  lateral  displacement ;  2d.  That  the  resistance  of  piles 
driven  into  heavy  solid  ground  is  much  greater  than  that  of 
piles  driven  into  sandy  ground ;  but  that  even  in  the  former 
case  the  piles  must  be  driven  rather  closely  if  they  are  to 
afford  an}^  efficient  resistance  to  small  lateral  displacements 
of  the  permanent  way-structure  ;  3d.  The  resisting  power  of 
piles,  and  especially  their  resistance  to  small  displacements, 
increasing  with  their  length,  and  in  a  more  rapid  ratio  than 
the  latter^  it  is'  considered  that  no  piles,  to  produce  an  effect 


448 


CIVIL  ENGIXEEEIXG. 


commensurate  with  their  cost,  should  have  a  length  of  less 
tlian  5  ft. ;  and  5th.  The  signs  of  considerable  displacements 
of  piles  maj,  under  certaiii  circumstances,  disappear  after 
the  causes  of  these  displacements  have  been  removed,  ^Vith- 
ont,  however,  the  piles  regaining  their  former  stability. 

759.  Experiments  to  determine  the  power  of  perma- 
nent way-structures  to  resist  the  loosening  of  tiie  rails 
from  the  sleepers.  It  is  remarked  by  Baron  von  Weber 
that  in  investigating  the  stability  of  the  connection  between 
rails  and  sleepers,  it  has  to  be  borne  in  mind  that  the  re- 
sistance of  the  rails  to  displacement  depends  upon  three 
things,  viz.  :  First,  the  holding  power  of  the  fastenings 
(bolts,  spikes,  etc.,  etc.)  by  which  the  rails  are  secured  to  the 
sleepers  ;  second,  to  the  increased  friction  between  the  base 
of  the  rails  and  the  sleepers  which  is  caused  by  a  load  stand- 
ing on  the  rails  ;  and,  third,  by  the  friction  between  the  rails 
and  the  wheels,  this  friction  causing  the  axles  to  form  ties  be- 
tween the  two  lines  of  rails  on  which  their  wheels  rest.  It 
will  thus  be  seen  that  the  gauge  of  a  line  of  rails  is  pre- 
served not  merely  by  the  fastenings  securing  the  rails  to  the 
sleepers,  but  also  by  other  forces  of  considerable  importance 
acting  both  on  the  top  and  bottom  of  the  rails. 

The  passage  of  the  rolling  stock  is  considered  by  Baron 
von  Weber  to  produce  on  the  rails  the  following  effects : — 

1.  Under  all  circumstances  a  vertical  pressure  tending  to 
force  the  rails  into  the  sleepers,  the  latter  yielding  to  this 
force  in  all  cases  where  they  are  not  made  of  materials  of 
very  high  resisting  powers,  such  as  stone  or  iron.  Wooden 
sleepers  are  of  course  compressed  by  the  vertical  pressure  of 
the  trains,  and  one  point  to  be  determined,  therefore,  is — 

760.  {ej  To  what  extent  are  sleepers  of  various  forms  and 
materials  compressed  hy  the  loads  acting  on  the  rails  f 

2.  There  is  a  horizontal  pressure  resulting,  in  the  case  of 
curves,  partly  from  centrifugal  force  and  partly  from  the 
rigidity  of  the  rolling  stock,  and,  in  the  case  of  straight  lines, 
from  the  oscillation  of  the  vehicles.  This  horizontal  pres- 
sure— which  may,  however,  change  into  a  pressure  acting  at 
a  more  or  less  acute  angle  to  the  surface  of  the  sleepers — 
tends  to  alter  the  position  of  the  rail  on  the  sleeper  in  two 
ways,  namely :  first,  by  shifting  the  rail  on  the  sleeper  with- 
out altering  the  inclination  of  the  former ;  and,  second,  by 
canting  the  rail  and  causing  it  to  turn  on  a  point  situated 
more  or  less  near  to  its  outer  ed^e,  according  to  the  com- 
pressibility of  the  sleeper.  The  first  of  these  two  kinds  of 
displacement  is  resisted  by  the  horizontal  resistance  of  the 


EAILWAYS. 


449 


spikes  or  other  fastenings,  by  the  friction  between  the  wheel 
and  the  rail,  and  by  the  friction  between  the  base  of  the  rail 
and  the  sleeper,  and  the  question  to  be  answered  by  the  ex- 
periments relating  to  this  kind  of  displacement,  therefore, 
is — 

761.  ( /)  What  power  is  required  to  displace  a  fastened 
and  loaded  rail  horizontally  on  its  sleepers  f 

The  second  kind  of  displacement  just  mentioned,  or  cant- 
ing of  the  rails  outwards,  is  resisted  by  the  direct  holding 
power  of  the  fastenings  connecting  the  rail  to  the  sleeper, 
and  by  the  friction  between  the  wheel  and  rail.  The  ques- 
tions to  be  answered  by  the  investigations  relating  to  this 
matter,  therefore,  are — 

{g)  What  force  is  required  to  draw  the  spikes  out  of  the 
sleepers?  and 

(A)  What  force  is  required  to  overcome  the  combined  re- 
sistance due  to  the  holding  power  of  the  spikes  in  the  sleep- 
ers, and  the  friction  between  the  rails  and  wheels? 

The  following  sets  of  experiments  were  carried  out  by 
Baron  von  Weber,  in  order  to  obtain  answers  to  the  above 
questions : — 

The  most  striking  result  obtained  is  the  deterioration  of 
the  sleepers  under  the  influence  of  the  traffic  at  the  points 
where  the  rails  rest  upon  them.  Thus  it  will  be  seen  that 
in  the  case  of  the  fir  sleepers  the  average  compressions  under 
the  load,  at  the  unused  and  old  bearing  surfaces  respectively, 
were  5.3  and  9,7  mils. ;  while  the  average  permanent  com- 
pressions were  1.1  and  2.6  mils.,  the  latter  results  being 
about  double  the  former. 

Another  remarkable  result  is  the  actual  amount  of  the 
compression,  this  amount  averaging  as  much  as  5.3  millime- 
tres (=  0.208  in.)  for  new  and  sound  fir  sleepers,  and  9.7  mil- 
limetres (—0.382  in.)  for  fir  sleepers  averaging  five  years  old. 
Baron  von  Weber  considers  that  these  results  point  to  the 
necessity  of  employing  rigid  rails,  so  as  to  distribute  the 
effects  of  the  pressure  of  the  rolling  stock  as  far  as  possible 
over  a  number  of  supports,  and  that  they  also  show  the  ad- 
vantage of  employing  sleepers  of  hard  timber. 

The  results  uf  the  first  group  of  experiments  relating  to 
question  {e)  Baron  von  Weber  summarizes  as  follows : — 

1.  That  sound  fir  sleepers  140  millimetres  (=5.5  in.)  thick 
and  200  millimetres  (=7.87  in.)  wide  are  compressed,  on  an 
average,  one  millimetre  (0.039  in.)  by  a  load  of  5.6  kilogram- 
mes per  square  centimetre  (=79.6  lb.  per  square  inch),  it 
being  supposed  that  they  have  not  before  been  subjected  to 
29 


460 


CIVIL  ENGINEERING. 


such  a  load.  At  places  where  rails  have  already  been  bear- 
ing upon  the  sleepers  for  some  time,  this  compression  is  in- 
creased to  one  millimetre  for  each  load  of  4  kilogrammes 
per  square  centimetre  (  =  56.88  lb.  per  square  inch). 

2.  The  action  of  the  trains  increases  considerably  the  com- 
pressibility of  the  sleepers  at  the  points  where  the  rails  bear 
upon  them. 

3.  That  the  compressibility  of  wooden  sleepers — and  espe- 
cially of  fir  sleepers — is  so  great,  that  it  is  necessary  to  dis- 
tribute the  pressure  of  the  trains  upon  the  sleepers  as  far  as 
possible  by  the  employment  of  rigid  rails. 

,  4.  That  increasing  the  number  *of  sleepers  in  order  to  in- 
crease the  carrying  power  of  a  permanent  way,  is,  theore- 
tically and  economically,  a  wrong  mode  of  obtaining  that 
end. 

5.  That  in  the  event  of  lateral  pressure  being  brought  to 
bear  against  the  head  of  the  rail,  the  resisting  power  of  fir 
sleepers  is  not  sufficiently  gr§at  to  prevent  a  canting  of  the 
rail  consequent  upon  the  impression  of  one  side  of  the  base 
into  the  sleeper.  Hence  momentary  alterations  in  the  gauge 
are  allowed,  these  alterations  disappearing,  however,  on  the 
removal  of  the  lateral  pressure,  and  leaving  no  traces  on  the 
spikes,  sleepers,  or  rails. 

6.  The  compression  of  fir  sleepers  under  the  bases  of  the . 
rails  is  so  great  that  the  cellular  structure  of  the  wood  is 
slowly  desti'oyed,  and  a  cutting  or  indentation  of  the  sleepers 
at  the  points  of  bearing  takes  place,  this  action  being  accele- 
rated when  the  upper  fibres  of  the  wood  have  been  more  or 
less  deprived  of  their  elasticity  by  the  action  of  the  weather. 

The  above  conclusions  are  justified  by  Baron  von  Weber's 
further  investigations. 

Baron  von  AV'eber's  deductions  from  the  second  group  of 
experiments  relating  to  question  (e)  are  as  follows: — 

1.  When  the  influence  of  the  rigidity  of  the  rail,  etc.,  upon 
the  transference  of  the  pressure  of  the  rolling  load  to  the 
sleeper  is  taken  into  account,  it  may  be  considered  that  the 
compression  of  the  sleeper  itself  takes  place  in  the  same 
manner  under  the  action  of  either  a  steadily  applied  or  a  roll- 
ing load. 

2.  That  the  sinking  of  well-bedded  sleepers  into  the  ground 
on  which  they  rest  is  proportionately  insignificant  even  under 
the  action  of  considerable  rolling  stock. 

3.  That  if  the  base  of  the  rail  has  a  bearing  surface  of  220 
square  centimetres  (=  32.1  square  inches)  upon  a  sound  fir 
sleeper  between  four  and  six  years  old,  and  140  millimetres 


RAILWAYS. 


451 


(=  5.5  inches)  thick,  a  load  of  1,500  kilogrammes  applied 
through  the  rail  will  compress  the  sleeper  one  millimetre. 
Or,  in  other  words,  a  load  of  about  7  kilogrammes  per  square 
centimetre  (=  99.54  lb.  per  square  inch)  will  produce  the 
compression  just  mentioned  on  those  parts  of  the  sleepers 
which  have  already  been  frequently  exposed  to  that  during  a 
considerable  time. 

Although  the  series  of  experiments  we  have  just  described 
are  not  extensive,  Baron  von  Weber  considers  that  the  follow- 
ing deductions  may  be  drawn  from  them :  1st.  That  the  re- 
sistance of  the  spikes  to  the  horizontal  displacement  of  the 
rails  upon  the  sleepers  is  proportionately  so  insignificant  that 
most  of  the  movements  of  the  rolling  stock  which  would  be 
capable  of  producing  a  displacement  of  the  rails  on  the 
sleepers  would  sufiice  to  overcome  this  resistance  ;  and,  2d. 
That  the  power  of  resistance  of  the  spikes  to  horizontal  dis- 
placement decreases,  after  that  displacement  has  once  begun, 
in  a  more  rapid  ratio  than  the  displacement  itself  increases ; 
and  hence  that  the  continued  action  of  the  rolling  stock  will 
produce  generally  greater  displacements  than  a  sudden  and 
great  pressure  or  force, 

762.  Herr  Funk's  Experiments  on  the  Resisting  Power 
of  Railway  Spikes.  The  experiments  made  by  Herr  Funk 
on  the  holding  power  of  railway  spikes  constitute,  as  we  re- 
marked, one  of  the  most  important  investigations  of  the  kind 
ever  carried  out,  the  experiments  being  directed,  not  merely  to 
ascertaining  the  power  of  the  spikes  to  resist  a  force  tending 
to  draw  them  straight  out  of  the  timber,  but  also  to  deter- 
mining their  resistance  to  lateral  displacement.  The  effect  of 
modifications  in  the  forms  of  the  spikes,  and  variations  in  the 
nature  of  the  timber  into  which  they  were  driven  were  also 
taken  into  consideration. 

The  resisting  power  of  railway  spikes  depends  chiefly — 

1.  Upon  the  kind  of  timber  of  which  the  sleeper  is  formed, 
and  the  condition  of  the  latter. 

2.  Upon  the  shape  and  dimensions  of  the  spikes. 

3.  Upon  the  mode  of  driving  them  into  the  sleepers. 

The  following  results  are  derived  from  the  above  investi- 
gations, and  from  former  experience  gained  in  the  construc- 
tion and  maintenance  of  permanent  way-structures : — 

1.  Sleepers  made  of  oak  are  preferable  to  those  made  of 
fir  and  deal,  not  only  on  account  of  their  greater  durability, 
but  also  on  account  of  the  greater  resisting  power  which  they 
give  to  the  spikes.  Although  experience  has  not  yet  suf- 
ficiently proved  the  proportionate  durability  of  prepared  oak, 


452 


CIVIL  ENGINEERING. 


fir,  and  pine  sleepers,  it  is,  nevertheless,  advisable  to  nse 
oak  sleepers,  even  in  cases  where  the  prices  for  the  oak 
are  1^  or  If  times  as  high  as  those  for  the  softer  kinds  of 
wood. 

2.  Joint  sleepers,  where  a  great  resisting  power  of  the 
spikes  is  especially  necessary,  ought  to  be  made  of  oak,  even 
in  tliose  cases  where  that  timber  costs  about  2  or  2J  times  as 
much  as  fir  or  pine.  If  the  difference  of  the  price,  how- 
ever, is  still  greater,  the  joint  sleepers  of  fir  ought  to  be 
made  larger,  in  order  to  enable  a  greater  resisting  power 
to  be  obtained  for  the  spikes  by  giving  the  latter  additional 
length. 

3.  If  the  intermediate  sleepers  are  made  of  fir,  one  or  two 
of  these  sleepers — according  to  whether  15  or  21  ft.  rails 
are  used— ought  to  have  two  spikes  on  the  outside  of  the  rail 
base,  or  small  bedplates,  3  or  4  inches  wide,  should  be  adopted, 
in  order  to  increase  the  resisting  power  of  the  spikes  against 
lateral  pressure,  and  especially  to  bring  the  inside  spike  also 
into  action.  The  number  of  these  outside  spikes  or  bedplates 
ought  to  be  increased  in  curves  of  small  radii  on  the  outer 
line  of  rails,  or  ought  to  be  provided  with  a  bedplate  with 
two  holes. 

4.  The  impregnation  of  the  sleepers  with  chloride  of  zinc 
does  not  influence  the  resisting  power  of  the  spikes,  but  this 
power  seems  to  be  a  little  less  for  newly  prepared  sleepers 
which  are  still  completely  saturated  with  water. 

6.  The  bellied  spikes  possess  the  smallest  resisting  power, 
this  power  being  only  0.7  or  0.9  of  that  for  prismatic  spikes 
of  the  same  weight. 

6.  Ko  favorable  result  is  obtained  by  twisting  the  spikes  or 
by  jagging  their  edges. 

7.  The  resisting  power  of  double  pyramidal  spikes  of  short 
length  is  for  deal  about  J  greater  than  that  of  straight  pris- 
matic spikes  of  the  same  weight;  this  advantage  does  not 
exist,  however,  in  the  case  of  spikes  of  greater  length,  nor 
when  the  spikes  are  driven  into  oak. 

8.  The  simple  pyramidal  spikes  and  the  prismatic  spikes,  if 
both  are  driven  equally  deep  into  the  wood,  offer  the  same  re- 
sisting power  against  being  drawn  out  of  the  timber,  whilst, 
if  the  same  volume  of  both  is  driven  into  the  wood,  the 
holding  power  of  the  former  is  for  oak  and  for  long  spikes 
about  -^jy,  and  for  deal  and  for  shorter  spikes  about  i 
greater  than  the  resisting  power  of  prismatic  spikes.  But 
with  respect  to  the  resisting  power  against  lateral  displace- 
ments within  the  limits  important  for  permanent  way-struc- 


RAILWATB. 


453 


tures,  the  prismatic  spikes  are  in  a  similar  proportion  stronger 
than  pyramidal  spikes. 

9.  The  pyramidal  spikes  costing  about  20  per  cent,  more 
than  prismatic  spikes  of  the  same  weight,  the  advantage  of 
the  smaller  volume  of  iron  driven  into  the  wood  for  the  ne- 
cessary depth  of  5  or  6  inches  (found  by  experience  to  be  a 
sufficient  depth  for  the  spiking  of  rails),  is  completely  com- 
pensated ;  the  prismatic  spikes  are,  therefore,  preferable  to 
pyramidal  spikes,  as  the  former,  besides  their  greater  resisting 
power  against  lateral  pressure,  have  not  the  great  disadvan- 
tage of  the  latter  spikes  of  becoming,  when  once  loosened, 
soon  entirely  powerless. 

763.  Baron  von  Weber's  Experiments  on  the  Re- 
sisting Power  of  Spikes.  The  experiments  above  de- 
scribed being  of  a  very  satisfactory  kind.  Baron  von  Weber's 
researches  were  conducted  so  as  to  deal  with  questions  to 
which  Herr  Funk's  experiments  did  not  relate,  and  they 
w^ere  especially  carried  out  for  the  purpose  of  ascertaining 
the  influence  of  the  pressure  of  the  wheels  against  the 
rails  upon  the  resisting  power  of  the  spikes. 

The  average  results  deduced  by  Baron  von  Weber,  from  the 
experiments  we  have  recorded,  are  that,  in  the  case  of  the  fir 
sleepers,  a  force  of  about  1,850  lbs.,  and  in  the  case  of  oak 
sleepers,  a  force  of  about  3,000  lbs.  was  required  for  drawing 
the  spikes.  As  the  latter  had  73  square  centimetres,  or  11.3 
square  inches,  of  surface  in  contact  with  the  timbers,  the 
forces  required  for  drawing  the  spikes  were  : 


Pounds  per  square  inch 
of  surface. 


In  fir  sleepers  163.2 

In  oak  sleepers  269.6 

These  values  for  the  holding  power  are  much  less  than  those 
found  by  von  Kaven  and  Funk,  and  there  is  also  somewhat 
less  difference  between  the  respective  holding  powers  in  fir 
and  oak  than  was  shown  by  the  researches  of  those  experi- 
menters. Baron  von  Weber,  however,  considers — and  we  agree 
with  him — that  the  difl'erence  between  von  Kaven  and  Funk's 
results  and  his  own  are  fully  accounted  for  by  the  fact  that 
in  the  latter  experiments  the  spikes  were  not  merely  subjected 
to  a  pull  in  the  direction  of  their  axes,  but  were  exposed  also 
!:o  lateral  pressure,  the  pull  being  exerted  on  the  underside  of 
the  nose  or  head.  Baron  von  Weber  considers  also  that,  from 
the  fibres  of  oak  having  less  flexibility  than  those  of  fir,  this 


/ 


464  CIVIL  ENGINEERING. 

lateral  pressure  would  produce  greater  loosening  of  the  spikes 
in  the  former  than  in  the  latter  timber,  and  hence  there  would 
be  less  difference  in  the  holding  power  of  the  spikes  in  the 
two  kinds  of  sleepers,  than  was  shown  by  the  researches  of 
former  experimenters,  who  applied  a  direct  pull  to  the  spikes. 

This  fact  shows,  as  is  remarked  by  Baron  von  Weber,  that 
results  of  direct  practical  value  can  only  be  obtained  by  ex- 
periments carried  out  under  the  circumstances  which  exist  in 
actual  practice,  and  he  considers,  for  this  reason,  that  the 
values  for  the  holding  power  of  spikes  deduced  from  his  re- 
searches are  more  reliable  for  practical  use  than  those  ob- 
tained from  previous  experiments. 

764.  Experiments  on  the  effects  of  Bedplates.  After 
the  preceding  experiments  had  been  carried  out,  it  became  de- 
sirable, in  order  to  complete  the  inquiries  relating  to  the  in- 
fluence of  the  means  usually  adopted  for  effecting  the  con- 
nection between  the  rails  and  sleepers,  that  some  experiments 
should  be  made  to  ascertain  the  effect  of  interposing  rolled 
iron  bedplates  between  the  sleepers  and  rails.  Such  bed- 
plates are  generally  supposed  to  serve  three  j)urposes.  Thus, 
first,  they  render  the  spikes  driven  into  the  sleepers  on  both 
sides  of  the  rail  dependent  on  each  other,  it  being  impossible 
for  one  to  be  displaced  without  the  other  being  displaced 
also  ;  and  thus  it  might  be  expected  d  jy^ioi'i  that  the  resist- 
ance of  the  spikes  to  lateral  displacement  would  be  doubled. 
Second,  the  plates  prevent  the  impression  of  the  edge  of  the 
rail  into  the  sleeper,  an  action  which  is  often  the  reason  for 
the  rail  canting ;  and,  third,  they  practically  increase  the 
bearing  surface  of  the  base  of  the  rail  upon  the  sleeper. 

In  this  series  of  trials,  two  pieces  of  rails  were  fastened,  at 
the  usual  gauge  apart,  upon  three  fir  sleepers,  and  between 
the  rails  and  the  central  sleepers  were  placed  bedplates  of  the 
Fig.  234.     Fig.  235.  shapc  sliowTL  iu  Figs.  234  and  *235.  The 
spikes  fitted  the  holes  in  the  plate  well,  and 
at  the  same  time  pressed  firmly  against 
the  bases  of  the  rails.    The  plates  were 
arranged  in  such  a  manner  that  the  side  of 
one  hole  was  placed  towards  the  inside  of 
the  rails,  and  the  press  acted  against  the 
heads    of   the  rails  directly   above  the 
plates. 

The  effect  of  the  plates  in  the  above  experiment  was  very 
clear,  and  they  evidently  increased  the  resistance  of  the  spikes 
to  lateral  displacement  until  the  latter  has  been  drawn  out  of 
the  timber.    In  fact,  the  pressure  required  to  loosen  the 


EAILWATS. 


455 


structure  was  more  than  double  that  necessary  in  the  case  of 
the  structure  without  plates. 

In  this  case,  the  rails  were  fixed  upon  two  sleepers,  bed- 
plates being  interposed  between  the  former  and  the  latter, 
and  the  press  being  placed  so  as  to  act  upon  the  heads  of  the 
rails  midway  between  the  two  sleepers. 

The  prevention  of  the  lateral  displacement  of  the  rails  re- 
sulting from  the  use  of  plates,  was  in  the  above  instance  the 
cause  of  a  greater  stability  of  the  heads  of  the  rails,  but  it  at 
the  same  time  had  the  effect  of  causing  the  more  rigid  struc- 
ture to  become  loosened  with  a  less  widening  of  the  gauge 
and  a  less  pressure  than  was  the  case  with  the  more  elastic 
structure  without  plates.  But  the  deferred  loosening  of  the 
structure  without  plates  was  practically  of  no  value,  for  be- 
fore the  loosening  took  place  the  gauge  had  been  widened  to 
such  an  extent  that  the  line  would  have  been  unfit  for  use. 

In  these  trials  the  rails  were  fastened  upon  four  sleepers  with 
bedplates,  and  the  press  acted  against  the  heads  of  the  rails 
in  the  middle  between  the  central  sleepers. 

The  loosening  of  the  structure  with  plates  took  place  at 
a  smaller  widening  of  the  gauge,  but  at  a  much  greater  pres- 
sure than  that  of  the  structure  without  plates;  and  the  resist- 
ance of  the  structure  was  in  fact  increased  by  the  use  of  the 
bedplates  more  than  60  per  cent. 

In  this  series  the  rails  were  fastened  down  to  five  sleepers, 
bedplates  being  interposed,  but  two  arrangements  of  the 
plates  were  tested.  In  the  first  case,  all  the  bedplates  were 
arranged  in  the  same  manner  as  in  the  previous  experiments, 
that  is,  with  the  side  traversed  by  one  spike  placed  inside ; 
but  in  the  second  case,  the  plates  on  the  three  central  sleepers 
were  turned  so  that  the  side  having  two  spikes  was  next  the 
centre  of  the  line.  Thus  six  extra  spikes  were  made  to  act 
against  the  canting  of  the  rails,  whilst  the  total  number  of 
spikes  securing  the  rails  to  the  sleepers  remained  the  same. 
The  second  arrangement  was  tested  for  the  purpose  of  ascer- 
taining the  most  advantageous  method  of  placing  the  plates 
to  secure  stability  of  the  structure. 

The  above  experiments  showed  that  the  stability  of  the  struc- 
ture was  practically  the  same  for  both  positions  of  the  plates, 
up  to  a  pressure  of  80  centners  (=  9,075  lbs.).  The  spikes 
in  the  normal  arrangements  then  became  loose,  while  the  other 
arrangement  with  two  spikes  inside  the  rails  on  each  of  the 
three  central  sleepers  allowed  a  further  widening  of  the 
gauge  ap  to  38  millimetres  (=1.496  in.)  before  the  resisting 
power  of  the  fastening  ceased.    The  second  arrangement  of 


456 


CrVTL  ENGINEERING. 


the  plates  thus  offered  a  greater  resistance  to  the  destruction  of 
the  structure  than  that  in  which  single  spikes  were  placed  in- 
side the  rails. 

765.  The  general  deductions  drawn  by  Baron  von  Weber 
from  all  the  experiments  relating  to  question  (^),  namely, 
What  force  is  required  to  draw  the  spikes  out  of  the  sleejpers  ? 
are  as  follows  : — 

1.  That  the  force,  in  pounds,  required  to  draw  out  of  tim- 
ber rail-spikes  of  the  usual  form — that  is  to  say,  square  pris- 
matic spikes  with  chisel  points — is  to  be  found,  if  the  strain 
acts  directly  in  the  direction  of  the  axis  of  the  spike,  by  mul- 
tiplying the  area  of  the  surface  of  the  spike  in  contact  with 
the  timber  by  the  following  numbers : — 

For  fir,   300  lbs.  ]  ( per  square  inch  af  surface  of  the  driven  portion  of 
"  oak,  GOO  "  f  "j     the  spike. 

"fir,  47  "  /  j  per  square  centimetre  of  surface  of  the  driven  portion 
*'  oak,  94   "  )  (    of  the  spike. 

If,  however,  the  force  acts  laterally  ^s>  well  as  in  the  direc- 
tion of  the  axis,  as  is  generally  the  case  in  practice,  the  mul- 
tipliers become  as  follows : — 

For  fir,   150  lbs.  )  j  per  square  inch  of  surface  of  the  driven  portion  of  the 
"  oak,  270    "   ]  {  spike. 

"fir,  25  "  )  (  per  square  centimetre  of  surface  of  the  driven  por- 
"  oak,    42   "  ]\     tion  of  the  spike. 

2.  That  spikes  driven  into  a  sleeper  for  the  second  time 
after  the  holes  in  the  timber  have  been  filled  up,  offer  at 
first  greater  resistance  than  spikes  driven  into  new  sleepers. 

3.  That  but  very  small  forces  are  required  to  produce  a 
widening  of  the  gauge  to  the  extent  of  6  or  10  millimetres 
(0.236  in.  or  0.394  in.)  as  such  amounts  of  widening  are  with- 
in the  limits  of  elasticity  of  the  structure,  and  require  no 
loosenings  of  the  fastenings. 

4.  That  a  lateral  pressure  of  80  centners  (=  9,075  lbs.)  at 
the  most,  acting  against  one  point  of  the  head  of  tlie  rails,  is 
Bufticient  to  produce  either  a  canting  or  lateral  displacement 
of  the  rails  to  such  an  extent  that  the  structure  at  this  point 
is  completely  and  permanently  loosened. 

5.  That  the  force  required  for  the  further  spreading  and 
final  destruction  of  the  structure  is  much  less  than  that  neces- 
sary for  the  first  loosening,  the  former  being  seldom  more 
than  75  per  cent,  of  the  latter. 

6.  That  the  resistance  of  the  structure  to  a  pressure  acting 
against  one  point  of  the  head  of  a  rail  does  not  increase  in 


RAILWAYS. 


457 


direct  proportion  to  the  number  of  sleepers  to  which  the  rail 
is  fastened,  but  that  the  elasticity  of  the  rail  and  consequent 
torsion  permits  the  fastenings  upon  the  several  sleepers  to  be 
loosened  successively.  The  resistance  of  the  rails  to  torsional 
strains  may,  however,  enable  the  fastenings  at  any  one  point 
to  receive  such  support  from  the  adjoining  fastenings  that  the 
resistance  to  canting  at  that  point  may  be  doubled. 

7.  That  if  the  elasticity  of  the  rails  is  very  great,  a  widen- 
ing of  the  gauge  to  the  extent  of  25  millimetres  (=0.984  in.) 
may  be  produced  without  remaining  permanent  or  without 
showing  signs  of  having  occurred  after  the  pressure  has  been 
removed.  This  is  more  likely  to  happen  if  the  widening  of 
the  gauge  is  produced  by  the  canting  of  the  rails  than  if  it  is 
due  to  their  lateral  displacement  on  the  sleepers ;  in  the  latter 
case  the  displacement  of  the  fastenings  would  be  visible, 
whilst  in  the  former  a  slight  raising  of  the  spikes  in  the  di- 
rection of  their  axis  would  only  be  observed  under  very 
favorable  circumstances. 

8.  That  in  the  case  of  structures  having  the  joints  of  the 
two  lines  of  rails  arranged  opposite  each  other  on  the  same 
sleeper,  the  points  on  which  the  joints  occur  offer  considera- 
bly less  resistance  to  a  widening  of  the  gauge  than  is  the  case 
when  the  rails  are  disposed  so  as  to  break  joint,  the  propor- 
tionate resisting  powers  in  the  two  cases  being  about  as  7 
to  10.  Thus  a  permanent  way,  having  the  joints  of  the  two 
lines  of  rails  opposite  each  other,  has  as  many  points  as  there 
are  joints,  at  which  the  lateral  stability  or  power  to  resist 
widening  of  the  gauge,  is  but  yV  of  that  at  the  joints 
of  the  structure  having  the  rails  disposed  so  as  to  break  joint. 
This  is  of  importance  with  respect  to  accidents  originating 
from  the  widening  of  the  gauge. 

9.  That  the  application  of  bedplates  between  the  rails  and 
sleepers  increases — under  otherwise  equal  circumstances — the 
power  of  resistance  of  the  structure  to  lateral  displacement 
of  the  rails ;  but  that  the  loosening  of  the  fastenings  takes 
place  with  a  smaller  displacing  movement. 

We  now  come  to  the  experiments  relating  to  question  (A), 
namely :  "  What  force  is  required  to  overcome  the  total  re- 
sistance due  to  the  combination  of  the  holding  power  of  the 
spikes  in  the  sleepers  and  the  friction  between  the  rails  and 
wheels  ? 

The  trials  just  recorded  are,  as  Baron  von  Weber  justly 
observes,  very  instructive,  for  they  prove  that  the  friction 
between  the  rails  and  the  sleejpers,  plus  the  resistance  of  the 
outside  spikes,  is  sufficient  to  keep  the  rails  in  their  places,  even 


458 


CIVIL  ENGINEERING. 


wlien  the  pressure  against  the  heads  is  such  as  to  cause  the 
canting  of  the  rails  to  an  extent  sufficient  to  render  the  line 
unfit  for  traffic.  The  experiments  also  show  that  the  inside 
spikes  afford  proportionately  little  resistance,  and  that  they 
represent  the  weakest  points  of  the  structure.  In  fact,  the 
fastened  and  loaded  rails  showed,  under  the  influence  of  the 
same  displacing  power,  displacements  which  were  certainly 
not  less  than  those  obtained  in  the  case  of  the  structure  in 
which  the  inside  spikes  had  been  loosened. 

Nothing  now  remained  connected  with  this  part  of  Baron 
von  Weber's  investigations  but  to  collect  facts  showing  the  in- 
fluence of  the  state  of  the  surface  of  the  rails  upon  the  stabil- 
ity of  the  structure. 

766.  The  deductions  made  by  him  from  the  experiments 
relating  to  the  question  (A),  "  What  force  is  required  to 
overcome  the  total  resistance  due  to  the  comhination  of  the 
holding  pov^er  of  the  sjpihes  in  the  sleepers  and  the  friction 
hetween  the  rails  and  wheels? "^^  are  as  follows: — 

1.  That  the  resisting  power  of  the  rails  to  lateral  forces  is 
considerably  increased  through  the  friction  between  the 
wheels  and  rails,  this  friction  causing  the  axle  to  form  a  kind 
of  tie  between  the  two  rails. 

2.  That  if  the  load  upon  the  rail  is  more  than  9,075  lbs. 
per  wheel  or  vehicle,  the  resisting  power  of  the  rails  to 
canting  due  to  the  friction  just  mentioned  is  greater  than 
that  due  to  the  spiking  of  the  rails  in  the  ordinary  w^ay  to  fir 
sleepers. 

3.  That  the  resistance  of  the  rails  to  lateral  displacement 
on  the  sleepers  is  increased  by  the  load  on  the  rails  in  the 
proportion  of  0.33  of  that  load  ;  whence,  in  the  case  of  rails 
carrying  the  load  of  6,806  lbs.  per  wheel,  the  resistance  of  the 
rails  to  lateral  displacement  on  the  sleepers  due  to  the  load, 
is  greater  than  that  due  to  the  resisting  power  of  the  spikes. 

4.  That  if  the  load  be  more  than  9,075  lbs.  per  wheel,  the 
frictional  resistances  cause  the  rails  to  be  supported  at  top 
and  bottom  against  both  canting  and  lateral  displacement, 
and  the  support  thus  afforded  is  more  effective  than  that  due 
to  the  ordinary  spiking. 

5.  That  the  forces  tending  to  produce  canting  and  lateral 
displacement  due  to  the  horizontal  oscillations  of  the  rolling 
stock,  can  only  be  resisted  (at  least  in  most  cases)  by  the  com- 
bined action  of  the  spikes,  the  friction  between  the  wheels 
and  rails,  and  the  friction  between  the  rails  and  sleepers. 

6.  That  if,  therefore,  the  load  upon  one  point  of  the  struc- 
ture be  partially  or  entirely  removed  by  the  undue  vertical 


RAILWAYS. 


459 


oscillation  of  a  vehicle,  whilst,  at  the  same  time,  a  lateral 
oscillation  of  the  vehicle  takes  place,  the  stability  of  the 
structure  against  the  pressure  due  to  this  lateral  oscillation 
depends  solely  upon  the  insufficient  resisting  power  of  the 
spikes,  and  the  lateral  distortion  and  displacement  are  the  un- 
avoidable consequences.  This  last  deduction  is,  as  Baron 
von  Weber  justly  considers,  one  of  very  great  importance, 
and,  in  fact,  the  experimental  researches  upon  which  it  is 
founded  may  be  said  to  prove  the  cause  which  leads  to  the 
serpentine  displacements  of  the  rails  but  too  frequently  met 
with  on  straight  portions  of  a  line  of  railway,  particularly  if 
the  line  is  one  of  light  construction,'  or  is  traversed  by  loco- 
motives having  considerable  overhang  at  the  leading  and 
trailing  ends.  If  such  a  portion  of  a  line  contains  a  sleeper 
badly  l3edded,  which  sinks  uniformly  throughout  its  entire 
length  under  the  influence  of  a  passing  load,  the  vehicle  pass- 
ing over  it  will  make  but  a  heavy  vertical  oscillation,  having 
no  influence  upon  the  lateral  resisting  power  of  the  structure. 
But  if  the  sleeper  sinks  under  one  rail  more  deeply  than  un- 
der the  other,  the  oscillation  of  the  vehicle  will  be  at  once 
horizontal  and  vertical,  and  the  load  will  be  removed  more  or 
less,  first  from  the  trailing  and  then  from  the  leading  axle, 
thus  causing  the  lateral  pressure  due  to  the  horizontal  oscilla- 
tions to  be  exerted  through  the  tires  of  the  wheels  with  full 
power  against  the  rails. 

In  such  a  case  it  is  almost  unavoidable  that  the  point  of 
the  unloaded,  or  partially  unloaded,  structure  should  be  dis- 
placed laterally ;  but  this  displacement  having  once  occurred, 
the  oscillations  of  the  passing  vehicles  become  so  consider- 
able, both  in  a  horizontal  and  vertical  direction,  that  the  dis- 
placement of  the  rail  is  soon  repeated,  and  only  favorable 
circumstances,  such  as  coincidence  of  the  oscillations,  can 
then  produce  a  uniform  motion  of  the  vehicles.  The  dis- 
placements just  referred  to  are  considered  by  Baron  von 
Weber  to  be  most  dangerous,  both  for  the  stability  of  the 
structure,  and  the  passage  of  the  trains,  because  their  original 
causes  can  only  be  discovered  with  great  difficulty,  even 
when  the  permanent  way  is  most  carefully  maintained. 

767.  l^otwithstanding  the  great  value  of  the  results  ob- 
tained from  the  experiments  we  have  already  described,  it  is 
undeniable  that  some  of  the  main  questions  relating  to  the  sta- 
bility of  permanent  way-structures  can  only  be  finally  an- 
swered by  ascertaining  the  amount  of  the  momentary  deflec- 
tions and  displacements  of  the  rails  which  actually  occur  when 
a  line  is  subjected  to  the  action  of  passing  trains,  lut  which 


460 


CIVIL  ENGINEERING. 


disappear  either  entirely,  or  almost  entirely,  after  the  action 
which  causes  them  ceases,  and  which  are  thus,  under  ordinary 
circumstances,  likely  to  escape  observation. 

The  momentary  deflections  and  displacements  just  referred 
to  may  be  divided  into  two  classes,  namely,  those  which  ap- 
parently disappear  on  the  removal  of  the  load,  and  those 
which  disappear  absolutely.  To  the  first  class  belong  those 
deflections  and  displacements  which,  although  causing  a 
greater  or  less  loosening  of  the  structure,  are  yet  within  the 
limits  of  elasticity  of  the  rails,  so  that  the  latter,  after  the  pas- 
sage of  the  train,  return  to  their  normal  positions,  and  there 
are  only  left  to  make  the  movements  which  have  taken  place^ 
the  small  lateral  displacements  of  the  spikes,  or  small  impres- 
sions of  the  sleepers  by  the  bases  of  the  rails.  Such  marks  of 
displacements  are  likely  to  escape  any  but  very  careful  in- 
spection ;  yet,  taken  altogether,  they  may  allow  to  the  rails 
an  amount  of  play  or  liberty  to  cant  which  may  produce 
dangerous  results.  The  second  class  of  momentary  displace- 
ments, on  the  other  hand,  consists  of  those  which  take  place 
within  the  limits  of  elasticity  of  the  permanent  way-structure 
as  a  whole,  all  the  parts  returning  to  their  normal  positions  on 
the  removal  of  the  cause  of  the  disturbance.  Such  momen- 
tary alterations  as  these  in  the  positions  of  the  rails  occur  less 
frequently  than  those  of  the  former  class,  but  they  may  never- 
theless become  dangerous  under  certain  circumstances  which 
will  be  spoken  of  hereafter. 

We  now  come  to  the  deductions  drawn  by  Baron  von  Weber, 
from  the  results  of  the  various  series  of  experiments  recorded 
by  us  in  the  preceding  articles  of  the  present  series.  It  is 
the  opinion  of  the  Baron  that  the  tendency  of  advanced  rail- 
way practice  is  to  abandon  the  ordinary  system  of  iron  or 
steel  rails  fixed  on  wooden  sleepers  for  the  use  of  permanent 
way-structures  formed  of  iron  alone,  and  he  considers  that 
ultimately  lines  of  rails  will  be  constructed  as  continuous  gir- 
ders, strong  enough  to  resist  all  the  actions  of  the  rolling 
stock,  and  resisting  directly  upon  properly  prepared  ground, 
without  the  intervention  of  intermediate  members  or  perish- 
able materials.  "  Looking  back,"  he  says,  "  upon  the  experi- 
mental researches,  we  are  struck  by  an  extraordinary  fact,  the 
remarkable  character  of  which  is  enlianced  by  the  circum- 
stance that  it  Jias  been  little  known  and  still  less  taken  into 
consideration.  This  fact  is  that  heavy  trains  and  powerful 
engines  have  already  ran  longer  than  the  age  of  the  present 
generation  upon  lines  or  structures,  the  flexibility  of  which  is 
BO  great  that  every  wheel  leaves  its  impression,  and  every  os- 


RAILWAYS. 


461 


cillation  produces  a  displacement ;  and  of  wliich  the  stability 
— as  far  as  it  depends  upon  the  resisting  power  of  its  mechan- 
ical parts — is  so  small  in  proportion  to  the  disturbing  influ- 
ences brought  to  bear  upon  it,  that  almost  any  one  of  these 
influences  Avould  destroy  the  structure  if  it  were  not  that  the 
very  load  itself  increased  the  stability  through  the  agency  of 
the  friction  between  the  wheels  and  the  rails.  It  would  be 
quite  unworthy  of  engineers  and  engineering  science  to  reply 
that  as  the  traflic  has  for  a  long  period  been  satisfactorily  car- 
ried on  lines  possessing  such  flexibility,  that,  therefore,  it  is  of 
no  importance  whence  the  stability  comes,  so  long  as  it  is  there 
when  required.  We  might  as  well  state  that  the  neighborhood 
of  a  certain  powder-mill  is  free  from  danger,  because  explo- 
sions have  occurred  but  rarely  during  the  last  five-and-thirty 
years." 

763.  Deductions  of  Baron  von  Weber  from  tabulated 
results.  Baron  von  Weber  makes  a  series  of  deductions  which 
are  worthy  of  the  careful  attention  of  both  locomotive  superin- 
tendents and  engineers  in  charge  of  permanent  way.  These 
deductions  are  in  substance  as  follows  : — 

1.  That,  as  is  well  known,  six-wheeled  locomotives,  when 
running,  oscillate  round  their  central  axle,  a  dipping  or 
plunging  motion  taking  place  towards  the  leading  and  trail- 
ing end  alternately.  Thus  the  loads  upon  the  leading  and 
trailing  springs  vary  according  to  the  oscillations,  and  conse- 
quently the  pressures  exerted  by  the  leading  and  trailing 
wheels  upon  the  rails  vary  also. 

2.  That  in  the  case  of  engines  on  which  the  experiments 
were  made  the  greatest  load  imposed  in  this  manner  upon  the 
springs  exceeded  the  normal  load  by  103  per  cent,  (the  in- 
crease of  load  being  from  70  to  160  centners  per  wheel)  in 
the  case  of  the  leading  springs,  and  by  74  per  cent,  (the  in- 
crease being  from  115  to  200  centners  per  wheel)  in  the  case 
of  the  trailing  springs. 

3.  That  the  maximum  loads  just  mentioned  are  much 
greater  than  that  laid  down  by  the  rules  acknowledged  by 
German  railways,  namely,  a  maximum  of  130  centners  per 
wheel.  Thus  in  determining  the  strength  of  permanent  way- 
structures  this  great  increase  of  the  pressure  sometimes  exer- 
cised upon  the  rails  should  be  taken  into  consideration. 

4.  That  the  load  upon  the  springs  is  sometimes  reduced 
during  the  running  of  the  engine  to  about  7  per  cent,  of  the 
normal  load  (the  reduction  being  from  72  to  5  centners)  in 
the  case  of  the  leading  springs,  and  to  26  per  cent,  of  the 
normal  load  (from  114  to  30  centners)  in  the  case  of  the 


462 


CIVIL  ENGINEERING. 


trailing  springs.  The  decrease,  or  even  sometimes  the  almost 
entire  removal  of  the  load  from  the  leading  springs  is  sur- 
prising. The  experiments,  of  which  an  account  has  just  been 
given,  prove  that  the  permanent  way  is  momentarily  sub- 
jected to  far  greater  loads  than  it  is  ordinarily  supposed  to 
carry,  and  furtl  her  that  it  is  sometimes  almost  entirelv  relieved 
of  its  load  as  above  stated.  It  appears  also  certain  that  there 
exist  horizontal  oscillations  of  the  vehicles  produced  at  first 
by  partially  vertical  oscillations,  and  there  thus  exists  the 
greatest  probability  of  the  coincidence  of  such  a  relief  from 
load  as  has  just  been  mentioned,  with  a  horizontal  oscillation 
towards  the  rail  from  which  the  load  has  just  been  removed, 
the  result  being  a  displacement  of  the  permanent  w^ay,  as, 
under  the  circumstances  supposed,  the  opposition  offered  by 
the  latter  is  but  that  due  to  its  mechanical  structure.  The 
experiments  on  the  stability  of  permanent  way  already  de- 
scribed, together  with  the  investigations  of  the  variations  of 
load  on  the  wheels  of  the  engines,  explain  in  a  satisfactory 
manner  the  causes  of  many  cases  of  widening  of  the  gauge 
and  displacement  of  the  structure  previously  considered 
inexplicable. 

5.  The  difference  between  the  maximum  and  minimum 
loads  resting  at  different  times  on  the  same  spring  varies  by 
more  than  double  the  normal  load  in  the  case  of  the  leading 
wheels ;  but  seldom  by  more  than  40  per  cent,  of  that  load 
in  the  case  of  the  trailing  wheels,  a  circumstance  which  indi- 
cates that  the  real  centre  of  oscillation  of  the  masses  forming 
the  engine  is  situated  between  the  driving  and  trailing  axle, 
and  not  over  the  former. 

6,  That  the  extreme  amounts  of  variation  in  the  loads  on 
the  leading  and  trailing  springs  were  found  to  occur  in  an 
engine  the  construction  of  which  would  have  least  justified 
the  expectation  of  their  taking  place.  This  engine  was  the 
"  Prometheus,"  in  which  the  wheel  base  differed  very  little 
from  the  length  of  the  boiler,  and  in  whicli  about  60  per 
cent,  of  the  load  was  removed  from  the  leading  wheel,  while 
that  on  the  trailing  wheels  was  reduced  to  77  per  cent,  of  the 
normal  load.  This  fact  points  strongly  to  the  danger  often 
attendant  upon  placing  a  great  load  upon  the  driving  axle, 
if  the  latter  is  situated  under  the  centre  of  the  engine. 

769.  Sleepers.  The  preservation  of  sleepers  by  chemical 
processes  is  always  the  subject  of  experiment  on  one  or  another 
of  our  railways.  The  practice,  however,  is  not  general  in 
this  country,  because  the  mashing  of  the  rail  into  the  sleeper 
usually  destroys  it  in  advance  of  decay.     In  England,  the 


EAILWATS. 


463 


bearings  of  the  chains  used  with  the  double-headed  rail  on 
every  sleeper  are  so  extended,  that  the  mechanical  injury  of 
the  wood  is  quite  small.  Prevention  against  decay — usually 
immei'sion  in  coal-tar — is  therefore  generally  practised.  The 
insufficient  bearing  offered  by  sleepers  to  the  rails  is  thus, 
directly  and  indirectly,  the  cause  of  their  rapid  destruction. 
It  is  stated  that  placing  the  sleepers  closer  than,  say  two  feet 
apart  between  centres,  would  prevent  the  convenient  tamping 
of  the  ballast.  It  is  objected  to  the  longitudinal  sleeper,  that  the 
rail  lying  parallel  with  the  fibre  of  the  wood,  mashes  into  it 
more  easily  than  into  the  cross-sleeper.  These  objections  to 
insufficient  bearing  are  not  inherent  in  either  system,  but  arise 
from  improper  construction.  Thoroughly  good  ballast  would 
not  require  continual  tamping.  It  is  even  proposed  b}^  some 
of  our  most  experienced  engineers  to  cover  the  ballast  with 
a  coating  of  coal-tar  and  gravel,  to  absolutely  exclude  water, 
and  thus  prevent  not  onl}'-  decay,  but- washing,  freezing,  heav- 
ing, settling — all  destroying  elements  but  vibration  and  wear. 
In  this  case  the  timber  bearings  under  the  rails  should  be 
almost  continuous,  to  prevent  wear  both  on  the  ballast  and 
on  the  rail.  The  mashing  of  rails  into  timbers,  either  longi- 
tudinals or  cross-sleepers,  is  largely  due  to  the  want  of  stiff- 
ness in  the  rails  themselves.  The  low  fl  rails  on  the  Great 
"Western  of  England  are  the  most  notable  examples  of  this 
kind  of  failure.  If  the  iron  wasted  in  the  thick  stem  and 
pear-head  of  our  worst  shaped  rails  were  put  into  the  height 
of  stem,  their  resistance  to  deflection  would  be  doubled,  this 
resistance  being  as  the  cube  of  the  depth. 

There  is  a  growing  conviction  among  engineers,  that  the 
longitudinal  system  will  become  standard.  It  offers  twice  to 
three  times  as  much  bearing  for  the  rail  as  the  cross-sleeper 
system.  The  whole  strength  of  a  longitudinal  is  added  to  the 
strength  of  the  rail,  considered  as  a  beam  to  carry  the  load. 
The  strength  of  the  cross- sleeper  in  this  direction  is  wholly 
wasted.  The  longitudinal  is  almost  certain  to  prevent  the 
displacement  of  a  broken  rail.  This  system  has  never  been, 
tried  on  a  large  scale,  with  a  high,  stiff  rail.  It  requires  bet- 
ter ballast,  and  more  thorough  adjustment  than  the  other 
system.  Independent  points  of  support,  like  the  isolated 
ends  of  cross-sleepers,  that  can  be  bloclved  up  or  let  down  at 
pleasure,  without  reference  to  the  rest  of  the  superstructure, 
are  the  indispensable  accompaniment  of  bad  ballasting  and 
imperfect  drainage.  But  they  are  unsuited  to  any  system  of 
homogeneous,  continuous,  and  permanent  way. 

Iron  sleepers  are  coming  into  use  in  countries  where  tim- 


464 


CIVIL  ENGINEERING. 


ber  is  very  costly  and  unsuitable,  and  are  the  subjects  of 
various  experiments  in  England. 

The  great  defect  of  all  imperishable  sleepers,  whether  stone 
or  iron,  has  been  want  of  elasticit}-.  An  anvil  under  a  rail, 
and  especially  under  a  joint,  is  as  bad  if  not  worse  than  an 
insufficient  support. 

770.  Rail- Joints.  The  selection  of  joint  fastenings  for 
the  ends  of  rails  is  somewhat  dependent  upon  the  weight  of 
rail  required,  and  hence  upon  the  traffic.  After  twenty  years 
of  competitive  trial  with  every  variety  of  fastening,  the  sim- 
ple "fish -joint" — an  iron  splice  on  each  side  of  the  rail — has 
become  standard  in  Europe,  and  is  gaining  ground  here.  It 
is  the  lightest  and  strongest  fastening  that  can  be  applied, 
when  rails  are  liigh,  and  properly  shaped  to  receive  it.  The 
old  difficulty  of  nuts  jarring  loose  has  been  overcome  by  the 
use  of  elastic  washers.  Fishing  a  pear-headed  rail,  three  or 
three  and  a  half  inches  high,  would  be  perfectly  useless.  For 
light  rails,  and  for  steel  rails  (to  save  weakening  them  by 
punching),  and  as  an  auxiliary  to  the  fish-joint,  the  new 
Reeves'  fastening — a  light  clamp  upon  the  contiguous  flanges 
of  two  rails — is  coming  largely  into  use.  The  mere  chair  or 
seating  for  the  ends  of  rails  is  no  longer  considered  safe  nor 
economical  for  lines  of  heavy  traffic.  Although  there  is 
room  for  farther  experiment,  it  cannot  be  said  that  the  de- 
mand for  a  good  rail-joint  has  not  been  met. 

771.  Steel  Rails — The  Results.  Bessemer  steel  rails 
have  been  in  regular  and  extensive  use  abroad  over  ten  years. 
For  several  years  large  trial-lots  have  been  laid  on  various 
American  roads  having  heavy  traffic. 

772.  The  Wear  of  Steel  Rails.  As  no  steel  rails  are  re- 
ported to  have  worn  out  on  our  roads,  the  comparative  dura- 
bility of  steel  and  iron  cannot  be  absolutel}^  determined. 

A  great  number  of  instances  of  the  comparative  wear  of 
steel  were  cited.  In  one  case  twenty-three  iron  rails  had  been 
worn  out,  where  a  steel  rail,  laid  end  to  end  with  the  iron, 
was  not  yet  worn  down.  In  other  cases  the  wear  was  seven- 
teen to  one.  It  is  conceded  that  any  steel  rail  will  outlast 
six  iron  rails.  In  fact,  the  remarkable  wearing  qualities  of 
steel  rails  have  never  been  doubted  or  questioned. 

773.  Breakage  of  Steel  Rails.  Some  steel  rails  of  Eng- 
lish, French,  and  American  manufacture  have  broken  in 
service.  In  several  cases  the  cause  has  been  ascertained  by 
the  direct  analysis  of  the  broken  rail.  The  cause  was  phos- 
phorus. In  some  other  cases,  where  analyses  were  not  made, 
the  general  character  of  the  iron  used  has  been  ascer- 


RAILWAYS. 


465 


tained,  and  the  trouble  has  been  inferred  to  be  phosphorus, 
or,  in  some  cases,  an  excess  of  silicon.  It  is  well  known  to 
steel  makers  that  a  very  minute  proportion  of  phosphorus 
(above  0.2  per  cent.)  will  make  Bessemer  steel  brittle.  In 
other  cases  rails  have  broken  at  the  mark  of  the  gag,"  or 
instrument  for  straightening  the  rail  cold.  The  rails  had  not 
been  properly  hot-straightened,  or  were  finished  at  too  low 
a  heat.  More  rails  have  broken  through  punched  fish-bolt 
holes,  and  at  punched  nicks  in  the  flange,  than  at  any  other 
places.  Experiments  prove  that  punching  a  hole  in  a  steel 
rail  which  is  sufficiently  hard  to  wear  well,  weakens  it. 

In  the  absence  of  further  official  information,  it  is  fair  to 
assume  that  the  breakage  of  steel  rails  is  only  a  small  per- 
centage of  the  breakage  of  iron  rails.  Indeed,  the  latter  is 
of  daily  occurrence,  and  is  rarely  considered  by  the  public, 
except  when  lives  are  lost,  and  not  always  by  railway  man- 
agers when  they  make  contracts. 

774.  Tests  and  Improvements.  The  punching  of  steel 
rails  has  been  abandoned.  Several  kinds  of  power  and  hand 
drilling  machines  have  been  introduced,  that  do  the  work 
rapidly  and  well.  The  loss  from  the  neutral  axis  of  a  rail, 
of  the  little  material  necessary  to  let  a  bolt  through,  cannot 
sensibly  weaken  it.  To  prevent  the  rails  from  creeping,  the 
engineer  of  the  Pennsylvania  railway  pins  them  to  several 
sleepers  by  means  of  \  inch  holes  drilled  in  the  flange. 
There  are  also  other  and  better  devices  for  preventing  end 
movement,  which  do  not  weaken  the  rail  at  all.  The  grand 
advantage  of  steel,  for  service  under  concussion  and  wear,  is 
its  homogeneity.  Having  been  cast  from  a  liquid  state,  it  is 
sound  and  uniform,  and  free  from  the  laminations  and  layers 
of  cinder  and  numerous  welds  which  characterize  wrought 
iron,  especially  the  low  grades  of  wrought  iron  usually  put 
into  rails. 

775.  Improved  Traction  upon  Steel  Rails,  It  has  been 
too  much  the  practice  of  railway  managers  to  consider  only 
the  increased  durability  of  steel.  A  less  striking,  but  per- 
haps equally  important  advantage  is,  that  it  has  double  the 
strength  and  more  than  double  the  stiffness  of  iron. 

The  great  and  constant  resistance  to  traction,  and  the  wear 
and  tear  of  track,  wheels,  and  running  gear,  due  to  the  deflec- 
tion of  rails  between  the  sleepers  ana  the  perpetual  series  of 
resulting  concussions,  may  be  much  reduced,  or  practically 
avoided,  by  the  use  of  rails  of  twice  the  ordinary  stiffness ; 
in  such  a  case,  however,  reasonably  good  ballasting  and 
sleepers  would  be  essential.  When  a  whole  series  of  sleep- 
30 


460 


CIVIL  ENGrNEERING. 


ers  sinks  bodily  into  the  mnd,  the  consideration  of  deflection 
between  the  sleepers  is  a  premature  refinement.  If  the 
weight  of  steel  rails  is  decreased  in  proportion  to  their 
strength,  these  advantages  of  cheaper  traction  and  mainte- 
nance will  not,  of  course,  be  realized.  The  best  practice,  here 
and  abroad,  is  to  use  the  same  weight  for  steel  as  had  been 
formerly  employed  for  iron. 

776.  Steel-headed  Rails.  Many  attempts  have  been  made 
in  England,  on  the  Continent,  and  in  this  country,  to  produce 
a  good  steel-headed  rail,  and  not  without  success.  Puddled 
steel  heads  have  all  the  structural  defects  of  wrought  iron, 
as  they  are  not  formed  from  a  cast,  and  hence  homogeneous 
mass,  but  are  made  by  the  wrought-iron  process,  and  are,  in 
fact,  a  "  high"  steely  wrought  iron.  They  are,  however,  a 
great  improvement  upon  ordinary  iron,  although  probably 
little  cheaper  than  cast-steel  heads.  Rolling  a  plain  cast-steel 
slab  upon  an  iron  pile  has  not  proved  successful.  The  weld 
cannot  be  perfected  on  so  large  a  scale,  and  the  steel  peels 
off  under  the  action  of  car  wheels.  Forming  the  steel  slab 
with  grooves,  into  which  the  iron  w^ould  dovetail  when  the 
pile  was  rolled  into  a  rail,  has  been  quite  successful. 


CANALS. 


467 


QHAPTEE  YIII. 

CANALS. 

T77.  Canals  are  artificial  channels  for  water,  applied  to  the 
purpose  of  inland  navigation ;  for  the  supply  of  cities  with 
water ;  for  draining ;  for  irrigation,  &c.,  &c. 

778.  Navigable  canals  are  divided  into  two  classes:  1st. 
Canals  which  are  on  the  same  level  throughout  their  entire 
length,  as  those  which  are  found  in  low  level  countries. 
2d.  Canals  which  connect  two  points  of  different  levels,  which 
lie  either  in  the  same  valley,  or  on  opposite  sides  of  a  dividing 
ridge.  This  class  is  found  in  broken  countries,  in  which  it  is 
necessary  to  divide  the  entire  length  of  the  canal  into  several 
level  portions,  the  communication  between  which  is  effected 
by  some  artificial  means.  When  the  points  to  be  connected 
lie  on  opposite  sides  of  a  dividing  ridge,  the  highest  reach, 
which  crosses  the  ridge,  is  termed  the  summit  level, 

119.  1st  Class.  The  surveying  and  laying  out  a  canal  in  a 
level  country,  are  operations  of  such  extreme  simplicity  as  to 
require  no  particular  notice  in  this  place. 

The  cross  section  of  this  class  (Fig.  236)  presents  usually  a 


Fig.  236— Cross  section  of  a  canal  in  level  catting. 

A,  water-way, 

B,  tow-paths. 

C,  berms, 

D,  side-drains. 

E,  puddling  of  clay  or  sand, 

watsr-wat/,  or  channel  of  a  trapezoidal  form,  with  an  embank- 
ment on  each  side,  raised  above  the  general  level  of  the 
country,  and  formed  of  the  excavation  for  the  water-way. 
The  level,  or  surface  of  the  water,  is  usually  above  the  natural 
surface,  sufficient  thickness  being  given  to  the  embankments 
to  prevent  the  filtration  of  the  water  through  them,  and  to  re- 
sist its  pressure.  This  arrangement  has  in  its  favor  the  advan- 
tage of  economy  in  the  labor  of  excavating  and  embanking, 


468 


CrVIL  ENGINEERING. 


since  the  cross  section  of  the  cutting  may  be  so  calculated  as 
to  furnish  the  necessary  earth  for  the  embankment ;  but  it 
exposes  the  surrounding  country  to  injury,  from  accidents 
happening  to  the  embankments. 

The  relative  dimensions  of  the  parts  of  the  cross  section 
may  be  generally  stated  as  follows ;  subject  to  such  modifica- 
tions as  each  particular  case  may  seem  to  demand. 

The  width  of  the  water-way,  at  bottom,  should  be  at  least 
twice  the  width  of  the  boats  used  in  navigating  the  canal ;  so 
that  two  boats,  in  passing  each  other,  may,  by  sheering  to- 
wards the  sides,  avoid  being  brought  into  contact. 

The  depth  of  the  water-way  should  be  at  least  eighteen 
inches  greater  than  the  draft  of  the  boat,  to  facilitate  the 
motion  of  the  boat,  particularly  if  there  are  water-plants 
growing  on  the  bottom. 

The  side  slopes  of  the  water-way,  in  compact  soils,  should 
receive  a  base  at  least  once-and-a-half  the  altitude,  and  pro- 
portionally more  as  the  soil  is  less  compact. 

The  thickness  of  the  embankments,  at  top,  is  seldom  regu- 
lated by  the  pressure  of  the  water  against  them,  as  this,  in 
most  cases,  is  inconsiderable,  but  to  prevent  filtration,  which, 
were  it  to  take  place,  would  soon  cause  their  destruction.  A 
thickness  from  four  to  six  feet,  at  top,  with  the  additional 
thickness  given  by  the  side  slopes  at  the  water  surface,  will, 
in  most  cases,  be  amply  sufticient  to  prevent  filtrations.  A 
pathway  for  the  horses  attached  to  the  boats,  termed  a  tow- 
jpatk^  which  is  made  on  one  of  the  embankments,  and  a  foot- 
path on  the  other,  which  should  be  wide  enough  to  serve  as 
an  occasional  tow-path,  give  a  superabundance  of  strength  to 
the  embankments. 

The  tow-path  should  be  from  ten  to  twelve  feet  wide,  to 
allow  the  horses  to  pass  each  other  with  ease  ;  and  the  foot- 
path at  least  six  feet  wide.  The  height  of  the  surfaces  of 
these  paths,  above  the  water  surface,  should  not  be  less  than 
two  feet,  to  avoid  the  wash  of  the  ripple  ;  nor  greater  than 
four  feet  and  a  half,  for  the  facility  of  the  draft  of  the  horses 
in  towing.  The  surface  of  the  tow-path  should  incline  slightly 
outward,  both  to  convey  off  the  surface  water  in  wet  weather, 
and  to  give  a  firmer  footing  to  the  horses,  which  naturally 
draw  from  the  canal. 

The  side  slopes  of  the  embankment  vary  with  the  character 
of  the  soil :  towards  the  water-way  they  should  seldom  be  less 
than  two  base  to  one  perpendicular  ;  from  it,  they  may,  if  it  be 
thought  necessary,  be  less.  The  interior  slope  is  usually  not 
carried  up  unbroken  from  the  bottom  to  the  top ;  but  a  hori- 


CANALS. 


469 


zontal  space,  termed  a  "bench,  or  herm,  about  one  or  two  feet 
•wide,  is  left,  about  one  foot  above  the  water  surface,  between 
the  side  slope  of  the  water-way  and  the  foot  of  the  embank- 
ment above  the  berm.  This  space  serves  to  protect  the  upper 
part  of  the  interior  side  slope,  and  is,  in  some  cases,  planted 
with  such  shrubbery  as  grows  most  luxuriantly  in  aquatic 
localities,  to  protect  more  efficaciously  the  banks  by  the  sup-  / 
port  which  its  roots  give  to  the  soil.  The  side  slopes  are 
better  protected  by  a  revetement  of  dry  stone.  Aquatic  plants 
of  the  bulrush  kind  have  been  used,  with  success,  for  the 
same  purpose ;  being  planted  on  the  bottom,  at  the  foot  of 
the  side  slope,  they  serve  to  break  the  ripple,  and  preserve 
the  slopes  from  its  effects. 

The  earth  of  which  the  embankments  are  formed  should  be 
of  a  good  binding  character,  and  perfectly  free  from  vegetable 
mould,  and  all  vegetable  matter,  as  the  roots  of  plants,  &c. 
In  forming  the  embankments,  the  vegetable  mould  should  be 
carefully  removed  from  the  surface  on  which  they  are  to  rest ; 
and  they  should  be  carried  up  in  uniform  layers,  from  nine 
to  twelve  inches  thick,  and  be  well  rammed.  If  the  charac- 
ter of  the  earth,  of  which  the  embankments  are  foi-med,  is 
such  as  not  to  present  entire  security  against  filtration,  a  pud- 
dling of  clay,  or  fine  sand,  two  or  three  feet  thick,  may  be 
laid  in  the  interior  of  the  mass,  penetrating  a  foot  below  the 
natural  surface.  Sand  is  useful  in  preventing  filtration  caused 
by  the  holes  made  in  the  embankments  near  the  water  sur- 
face by  insects,  moles,  rats,  &c. 

Side  drains  must  be  made,  on  each  side,  a  foot  or  two  from 
the  embankments,  to  prevent  the  surface  water  of  the  natural 
surface  from  injuring  the  embankments. 

780.  2d  Class.  This  class  will  admit  of  two  subdivisions : 
1st.  Canals  which  lie  throughout  in  the  same  valley  ;  2d. 
Canals  with  a  summit  level. 

Location.  In  laying  out  canals,  belonging  to  the  first  sub- 
division, the  engineer  must  be  guided  in  his  choice  by  the 
relative  expense  of  construction  on  the  two  sides  of  the  valley ; 
which  will  depend  on  the  quantity  of  cutting  and  filling,  the 
masonry  for  the  culverts,  &c.,  and  the  nature  of  the  soil  as 
adapted  to  holding  water.  All  other  things  being  equal,  the 
side  on  which  the  fewest  secondary  water-courses  are  found 
will,  generally  speaking,  offer  the  greatest  advantage  as  to 
expense,  but  it  may  happen  that  the  secondary  water-courses 
will  be  required  to  feed  the  canal  with  water,  in  which  case 
it  will  be  necessary  to  lay  out  the  line  on  the  side  where  thev 
are  found  most  convenient,  and  in  most  abundance. 


470 


CrVTL  EKGINEEKESTG. 


781.  Cross  section.  The  side  formations  of  excavations 
and  embankments  require  peculiar  care,  particularly  the  lat- 
ter, as  any  crevices,  when  they  are  first  formed,  or  which  may 
take  place  by  settling,  might  prove  destructive  to  the  work. 
In  most  cases,  a  stratum  of  good  binding  earth,  lining  the 
w^ater-way  throughout  to  the  thickness  of  about  four  feet,  if 
compactly  rammed,  will  be  found  to  offer  sufficient  security, 
if  the  substructure  is  of  a  firm  character,  and  not  liable  to 
settle.  Fine  sand  has  been  applied  with  success  to  stop  the 
leakage  in  canals.  The  sand  for  this  purpose  is  sprinkled,  in 
small  quantities  at  a  time,  over  the  surface  of  the  water,  and 
gradually  fills  up  the  outlets  in  the  bottom  and  sides  of  the 
canal.  But  neither  this  nor  puddling  has  been  found  to  an- 
swer in  all  cases,  particularly  where  the  substructure  is  formed 
of  fragments  of  rocks  offering  large  crevices  to  filtrations,  or 
is  of  a  marly  nature.  In  such  cases  it  has  been  found  neces- 
sary to  line  the  water-way  throughout  with  stone,  laid  in  hy- 
draulic mortar.    A  lining  of  this  character  (Fig.  237),  both 


Fig.  237— Cross  section  of  a  canal  in  side  cutting  lined  with  masonry. 

A.  water-way. 

B,  tow-paths. 

D,  embankment. 
O,  masonry  lining. 


at  the  bottom  and  sides,  formed  of  flat  stones,  about  four  in- 
ches thick,  laid  on  a  bed  of  hydraulic  mortar,  one  inch  tliick, 
and  covered  by  a  similar  coat  of  mortar,  making  the  entire 
thickness  of  the  lining  six  inches,  has  been  found  to  answer 
all  the  required  purposes.  This  lining  should  be  covered,  both 
at  bottom  and  on  the  sides,  by  a  layer  of  good  earth,  at  least 
three  feet  thick,  to  protect  it  from  the  shock  of  the  boats 
striking  either  of  those  parts. 

The  cross  section  of  the  canal  and  its  tow-paths  in  deep  cut- 
ting (Fig.  238)  should  be  regulated  in  the  same  way  as  in 
canals  of  the  first  class ;  but  when  the  cuttings  are  of  consid- 
erable depth,  it  has  been  recommended  to  reduce  both  to  the 
dimensions  strictly  necessary  for  the  passage  of  a  single  boat. 


CANALS. 


Fig.  238— Cross  section  of  a  canal  in  deep  cutting. 
E,  side  slopes  of  cutting. 

By  this  reduction  there  would  be  some  economy  in  the  exca- 
vations ;  but  this  advantage  would,  generally,  be  of  too  tri- 
fling a  character  to  be  placed  as  an  offset  to  the  inconveni- 
ences resulting  to  the  navigation,  particularly  where  an  active 
trade  was  to  be  carried  on. 

782.  Summit  level.  As  the  water  for  the  supply  of  the 
summit  level  of  a  canal  must  be  collected  from  the  ground 
that  lies  above  it,  the  position  selected  for  the  summit  level 
should  be  at  the  lowest  point  practicable  of  the  dividing  ridge, 
between  the  two  branches  of  the  canal.  In  selecting  this 
point,  and  the  direction  of  the  two  branches  of  the  canal,  the 
engineer  will  be  guided  by  the  considerations  with  regard  to 
the  natural  features  of  the  surface,  which  have  already  been 
dwelt  upon. 

783.  Supply  of  water.  The  quantity  of  water  required 
for  canals  with  a  summit  level,  may  be  divided  into  two  por- 
tions. 1st.  That  which  is  required  for  the  summit  level,  and 
those  levels  which  draw  from  it  their  supply.  2d.  That 
which  is  wanted  for  the  levels  below  those,  and  which  is  fur- 
nished from  other  sources. 

The  supply  of  the  first  portion,  which  must  be  collected  at 
the  summit  level,  may  be  divided  into  several  elements :  1st. 
The  quantity  required  to  fill  the  summit  level,  and  the  levels 
which  draw  their  supply  from  it.  2d.  the  quantity  required 
to  supply  losses,  arising  from  accidents ;  as  breaches  in  the 
banks,  and  the  emptying  of  the  levels  for  repairs.  3d.  The 
supplies  for  losses  from  surface  evaporation,  from  leakage 
through  the  soil,  and  through  the  lock  gates.  4th.  The  quan- 
tity required  for  the  service  of  the  navigation,  arising  from 
the  passage  of  the  boats  from  one  level  to  another.  Owing 
to  the  want  of  sufficient  data,  founded  on  accurate  observa- 
tions, no  precise  amount  can  be  assigned  to  these  various  ele- 
ments which  will  serve  the  engineer  as  data  for  rigorous  cal- 
culation. 

The  quantity  required,  in  the  first  place,  to  fill  the  summit 
level  and  its  dependent  levels,  will  depend  on  their  size,  an 


472 


CIVIL  ENGINEERriTG. 


element  which  can  be  readily  calculated;  and  upon  the  quan- 
tity which  would  soak  into  the  soil,  which  is  an  element  of  a 
very  indeterminate  character,  depending  on  the  nature  of  the 
Boil  in  the  different  levels. 

The  supplies  for  accidental  losses  are  of  a  still  less  deter- 
minate charactei'. 

To  calculate  the  supply  for  losses  from  surface  evaporation, 
correct  observations  must  be  made  on  the  yeai-ly  amount  of 
evaporation,  and  the  quantity  of  rain  tliat  falls  on  the  sur- 
face ;  as  the  loss  to  be  supplied  will  be  the  difference  be- 
tween these  two  quantities. 

With  regard  to  the  leakage  through  the  soil,  it  will  depend 
on  the  greater  or  less  capacity  which  the  soil  has  for  holding 
water.  This  element  varies  not  only  with  the  nature  of  the 
soil,  but  also  with  the  shorter  or  longer  time  that  the  canal 
may  have  been  in  use ;  it  having  been  found  to  decrease  with 
time,  and  to  be,  comparatively,  but  trifling  in  old  canals.  In 
ordinary  soils  it  may  be  estimated  at  about  two  inches  in 
depth  every  twenty -four  hours,  for  some  time  after  the  canal 
is  first  opened.  The  leakage  through  the  gates  will  depend 
on  the  workmanship  of  these  parts.  From  experiments  by 
Mr.  Fisk,  on  the  Chesapeake  and  Ohio  canal,  the  leakage 
through  the  locks  at  the  summit  level,  which  are  100  feet 
long,  15  feet  wide,  and  have  a  lift  of  8  feet,  amounts  to 
twelve  locks  full  daily,  or  about  62  cubic  feet  per  minute. 
The  monthly  loss  upon  the  same  canal,  from  evaporation  and 
filtration,  is  about  twice  the  quantity  of  water  contained  in 
it.  From  experiments  made  by  Mr.  J.  B.  Jervis,  on  the  Erie 
canal,  the  total  loss,  from  evaporation,  filtration,  and  leakage 
through  the  gates,  is  about  100  cubic  feet  per  minute,  for 
each  mile. 

In  estimating  the  quantity  of  water  expended  for  the  ser- 
vice of  the  navigation,  in  passing  the  boats  from  one  level  to 
another,  two  distinct  cases  require  examination :  1st.  Where 
there  is  but  one  lock  between  two  levels,  or  in  other  words, 
when  the  locks  are  isolated.  2d.  When  there  are  several 
contiguous  locks,  or  as  it  is  termed,  2^  flight  of  locks  between 
two  levels. 

784.  A  loch  is  a  small  basin  just  large  enough  to  receive 
a  boat,  in  which  the  water  is  usually  confined  on  the  sides  by 
two  upright  walls  of  masonry,  and  at  the  ends  by  two  gates, 
which  open  and  shut,  both  for  the  purpose  of  allowing  the 
boat  to  pass,  and  to  cut  off  the  water  of  the  upper  level  from 
the  lower,  as  well  as  from  the  lock  while  the  boat  is  in  it.  To 
pass  a  boat  from  one  level  to  the  other— from  the  lower  to  the 


Fig.  239— Is  a  plan  of  the  present  enlarged  form  of  one-half  of  a  double  lock  on  the  Erie, 
Canal. 


m 


CIVIL  ENGINEERING. 


upper  end,  for  example— the  lower  gates  are  opened,  and  the 
boat  having  entered  the  lock  they  are  shut,  and  water  is  drawn 
from  the  upper  level,  by  means  of  valves,  to  till  the  lock  and 
raise  the  boat ;  wlien  this  operation  is  finished,  the  upper  gates 
are  opened,  and  the  boat  is  passed  out.  To  descend  from  the 
upper  level,  the  lock  is  first  filled  ;  the  upper  gates  are  then 
opened,  and  the  boat  passed  in ;  tliese  gates  are  next  shut,  and 
the  water  is  drawn  from  the  lock  by  valves,  until  the  boat  is 
lowered  to  the  lower  level,  when  the  lower  gates  are  opened 
and  the  boat  is  passed  out. 

In  the  two  operations  just  described,  it  is  evident,  that  for 
the  passage  of  a  boat,  up  or  down,  a  quantity  of  water  must 
be  drawn  from  the  upper  level  to  fill  the  lock  to  a  height 
which  is  equal  to  the  difference  of  level  between  the  surface 
of  the  water  in  the  two  ;  this  height  is  termed  the  lift  of  the 
lock,  and  the  volume  of  water  required  to  pass  a  boat  up  or 
down  is  termed  the  prism  of  lift.  The  calculation,  therefore, 
for  the  quantity  of  water  requisite  for  the  service  of  the  navi- 
gation, will  be  simply  that  of  the  number  of  prisms  of  lift 
which  each  boat  will  draw  from  the  summit  level  in  passing 
up  or  down. 

785.  In  calculating  the  expenditure  for  locks  in  flights,  a 
new  element,  termed  the  prism  of  draughty  must  be  taken  into 
account.  This  prism  is  the  quantity  of  water  required  to  float 
the  boat  in  the  lock  when  the  prism  of  lift  is  drawn  off  ;  and 
is  evidently  equal  in  depth  to  the  water  in  the  canal,  unless  it 
should  be  deemed  advisable  to  make  it  just  sufficient  for  the 
draught  of  the  boat,  by  which  a  small  saving  of  water  might 
be  effected. 

786.  Locks  in  flights  may  be  considered  under  two  points 
of  view,  with  regard  to  the  expenditure  of  water :  the  first, 
where  both  the  prism  of  lift,  and  that  of  draught,  are  drawn 
off  for  the  passage  of  a  boat ;  or  second,  where  the  prisms  of 
draught  are  always  retained  in  the  locks.  The  expenditure, 
of  course,  will  be  different  for  the  two  cases. 

Great  refinements  in  the  calculation  of  such  cases  should 
not  be  made,  but  the  engineer  should  confine  himself  to  mak- 
ing an  ample  allowance  for  the  most  unfavorable  cases,  both 
as  regards  the  order  of  passage  and  the  number  of  boats. 

787.  Feeders  and  Reservoirs.  Having  ascertained,  f  mm 
the  preceding  considerations,  tlie  probable  supply  which 
should  be  collected  at  the  summit  level,  the  engineer  will 
next  direct  his  attention  to  the  sources  from  which  it  may  be 
procured.  Theoretically  considered,  all  the  water  that  drains 
from  the  ground  adjacent  to  the  summit  level,  and  above  it, 


CANALS. 


475 


might  be  collected  for  its  supply  ;  but  it  is  found  in  practice 
that  channels  for  the  conveyance  of  water  must  have  certain 
slopes,  and  that  these  slopes,  moreover,  will  regulate  the  sup- 
ply furnished  in  a  certain  time,  all  other  things  being  equal. 
In  making,  however,  the  survey  of  the  country,  from  which 
the  water  is  to  be  supplied  to  the  summit  level,  all  the  ground 
above  it  should  be  examined,  leaving  the  determination  of  the 
slopes  for  after  considerations.  The  survey  for  this  object 
consists  in  making  an  accurate  delineation  of  all  the  water- 
courses above  the  summit  level,  and  in  ascertaining  the  quan- 
tity of  water  which  can  be  furnished  by  each  in  a  given  time. 
This  survey,  as  well  as  the  measurement  of  the  quantity  of 
water  furnished  by  each  stream,  which  is  termed  the  gauging, 
should  be  made  in  the  driest  season  of  the  year,  in  order  to  as- 
certain the  minimum  supply. 

788.  The  usual  method  of  collecting  the  water  of  the 
sources,  and  conveying  it  to  the  summit  level,  is  by  feeders 
and  reservoirs.  T\\q  feeder  is  a  canal  of  a  small  cross  section, 
which  is  traced  on  the  surface  of  the  ground  with  a  suitable 
slope,  to  convey  the  water  either  into  the  reservoir,  or  direct 
to  the  summit  level.  The  dimensions  of  the  cross  section, 
and  the  longitudinal  slope  of  the  feeder,  should  bear  certain 
relations  to  each  other,  in  order  that  it  shall  deliver  a  certain 
supply  in  a  given  time.  The  smaller  the  slope  given  to  the 
feeder,  the  lower  will  be  the  points  at  which  it  will  intersect 
the  sources  of  supply,  and  therefore  the  greater  will  be  the 
quantity  of  water  which  it  will  receive.  This  slope,  however, 
has  a  practical  limit,  which  is  laid  down  at  four  inches  in 
1,000  yards,  or  nine  thousand  base  to  one  altitude ;  and  the 
greatest  slope  should  not  exceed  that  which  would  give  the 
current  a  greater  mean  velocity  than  thirteen  inches  per  sec- 
ond, in  order  that  the  bed  of  the  feeder  may  not  be  injured. 
Feeders  are  furnished  like  ordinary  canals,  with  contrivances 
to  let  off  a  part,  or  the  whole,  of  the  water  in  them,  in  cases 
of  heavy  rains,  or  for  making  repairs. 

But  a  small  proportion  of  the  water  collected  by  the  feed- 
ers is  delivered  at  the  reservoir  ;  the  loss  from  various  causes 
being  much  greater  in  them  than  in  canals.  From  observa- 
tions made  on  some  of  the  feeders  of  canals  in  France,  which 
have  been  in  use  for  a  long  period,  it  aj)pears  that  the  feeder 
of  the  Briare  canal  delivers  only  about  one-fourth  of  the  water 
it  gathers  from  its  sources  of  supply  ;  and  that  the  annual  loss 
of  the  two  feeders  of  the  Languedoc  canal  amounts  to  100 
times  the  quantity  of  water  which  they  can  contain. 

789.  A  Reservoir  is  a  large  pond,  or  body  of  water,  held  in 


476 


CIVIL  ENGINEERING. 


reserve  for  the  necessary  supply  of  the  summit  level.  A  reser- 
voir is  usually  formed  by  choosing  a  suitable  site  in  a  deep 
and  narrow  valley,  which  lies  above  the  summit  level,  and 
erecting  a  dam  of  earth,  or  of  masonry,  across  the  outlet  of 
the  valley,  or  at  some  more  suitable  point,  to  confine  the 
water  to  be  collected.  The  object  to  be  attained,  in  this  case, 
is  to  embody  the  greatest  volume  of  water,  and  at  the  same 
time  present  the  smallest  evaporating  surface,  at  the  smallest 
cost  for  the  construction  of  the  dam. 

It  is  generally  deemed  best  to  have  two  reservoirs  for  the 
supply,  one  to  contain  the  greater  quantity  of  water,  and  the 
other,  which  is  termed  the  distributing  reservoir,  to  regulate 
the  supply  to  the  summit  level.  If,  however,  the  summit 
level  is  very  capacious,  it  may  be  used  as  the  distributing 
reservoir. 

The  proportion  between  the  quantity  of  water  that  falls 
upon  a  given  surface,  and  that  which  can  be  collected  from 
it  for  the  supply  of  a  reservoir,  varies  considerably  with  the 
latitude,  the  season  of  the  year,  and  the  natural  features  of 
the  locality.  The  drainage  is  greatest  in  high  latitudes,  and 
in  the  winter  and  spring  seasons ;  with  respect  to  the  natural 
features,  a  wooded  surface  with  narrow  and  deep  valleys  will 
yield  a  larger  amount  than  an  open  flat  country. 

But  few  observations  have  l)een  made  on  this  point  by  en- 
gineers. From  some  by  Mr.  J.  B.  Jervis,  in  reference  to  the 
reservoirs  for  the  Chenango  canal,  in  the  State  of  New  York, 
it  appears  that  in  that  locality  about  two-fifths  of  the  quan- 
tity of  rain  may  be  collected  for  the  supply  of  a  reservoir. 
The  proportion  usually  adopted  by  engineers  is  one-third. 

The  loss  of  water  from  the  reservoir  by  evaporation,  filtra- 
tion, and  other  causes,  will  depend  upon  the  nature  of  the 
soil,  and  the  exposure  of  the  water  surface.  From  observa- 
tions made  upon  some  of  the  old  reservoirs  in  England  and 
Francie,  it  appears  that  the  daily  loss  averages  about  half  an 
inch  in  depth. 

790.  The  dams  of  reservoirs  have  been  variously  con- 
structed :  in  some  cases  they  have  been  made  entirely  of 
earth  (Fig.  240) ;  in  others,  entirely  of  masonry ;  and  in 
others,  of  earth  packed  in  between  several  parallel  stone 
walls.  It  is  now  thought  best  to  use  either  earth  or  masonr}^ 
alone,  according  to  the  circumstances  of  the  case ;  the  com- 
parative expense  of  the  two  methods  being  carefully  con- 
sidered. 

Earthen  dams  should  be  made  with  extreme  care,  of  the 
best  binding  earth,  well  freed  from  everything  that  might 


CANALS. 


Fig.  240 — Represents  the  section  of  a  dam  with  three  discharging  culverts, 

A,  body  of  the  dam. 

B,  pond. 

a,  a,  a,  culverts,  with  valves  at  their  inlets,  which  discharge  into  the  vertical  well  &. 

c,  c,  c,  grooves,  in  the  faces  of  the  side-walls,  which  form  the  entrance  to  the  culverts,  for  stop- 
plank. 

d,  stop-plank  dam  across  the  outlet  of  the  bottom  culvert,  to  dam  back  the  water  into  the 
vertical  well. 

c,  parapet  waU  on  top  of  the  dam. 

cause  filtration  s.  A  wide  trench  should  be  excavated  to  the 
firm  soil,  to  receive  the  base  of  the  dam ;  and  the  earth  should 
be  carefully  spread  and  rammed  in  layers  not  over  a  foot 
thick.  As  a  farther  precaution,  it  has  in  some  instances  been 
thought  necessary  to  place  a  stratum  of  the  best  clay  pud- 
dling in  the  centre  of  the  dam,  reaching  from  the  top  to  three 
or  four  feet  below  the  base.  The  dam  may  be  from  fifteen 
to  twenty  feet  thick  at  top.  The  slope  of  the  dam  towards 
the  pond  should  be  from  three  to  six  base  to  one  perpendic- 
ular ;  the  reverse  slope  need  only  be  somewhat  greater  than 
the  natural  slope  of  the  earth. 

The  slope  of  dams  exposed  to  the  water  is  usually  faced 
with  dry  stone,  to  protect  the  dam  from  the  action  of  the 
surface  ripple.  This  kind  of  facing  has  not  been  found  to 
withstand  well  the  action  of  the  water  when  agitated  by  high 
winds.  Upon  some  of  the  more  recent  earthen  dams  erected 
in  France,  a  facing  of  stone  laid  in  hydraulic  mortar  has  been 
substituted  for  the  one  of  dry  stone.  The  plan  adopted  for 
this  facing  (Fig.  241)  consists  in  placing  a  series  of  low  walls, 


Fig.  241— "Represents  the  method  of 
facing  the  pond  slope  of  a  dam, 
with  low  walls  placed  in  ofEsets. 

A,  body  of  the  dam. 

a,  a,  a,  low  walls  the  faces  of  which 
are  built  in  offsets. 

6,  6,  top  surface  of  the  ofEsets  be- 
tween the  walls,  covered  with 
stone  slabs  laid  in  mortar. 

c,  top  of  dam  faced  like  the  ofEsets 
h. 

<!,  parapet  waU. 


478 


CIVIL  ENGINEERING. 


in  offsets  above  each  other,  along  the  slope  of  the  dam,  cover- 
ing the  exposed  surface  of  each  offset,  between  the  top  of  one 
wall  and  the  foot  of  the  next,  with  a  coating  of  slab-stone  laid 
in  mortar.  The  walls  are  from  live  to  six  feet  high.  They 
are  carried  up  in  small  offsets  upon  the  face,  and  are  made 
either  vertical,  or  leaning,  on  the  back.  The  width  of  the  off- 
sets of  the  dam,  between  the  top  of  one  wall  and  the  foot  of 
the  next,  is  from  two  to  three  feet. 

An  arched  culvert,  or  a  large  cast-iron  pipe,  placed  at  some 
suitable  point  of  the  base  of  the  dam,  which  can  be  closed  or 
opened  by  a  valve,  will  serve  for  drawing  off  the  requisite 
supply  of  water,  and  for  draining  the  reservoir  in  case  of  re- 
pairs. 

The  culvert  should  be  strongly  constructed,  and  the  earth 
around  it  be  well  puddled  and  rammed,  to  prevent  filtrations. 
Its  size  should  be  sufficient  for  a  man  to  enter  it  with  ease. 
The  valves  may  be  placed  either  at  the  entrance  of  the  cul- 
vert, or  at  some  intermediate  point  between  the  two  ends. 
Great  care  should  be  taken  in  their  arrangement,  to  secure 
them  from  accidents. 

When  the  depth  of  water  in  a  reservoir  is  considerable,  sev- 
eral culverts  should  be  constructed  (Fig.  240),  to  draw  off  the 
water  at  different  levels,  as  the  pressure  upon  the  lower  valves 
in  this  case  would  be  very  great  when  the  reservoir  is  full. 
They  may  be  placed  at  intervals  of  about  twelve  feet  above 
each  other,  and  be  arranged  to  discharge  their  water  in  a  com- 
mon vertical  shaft.  In  this  case  it  will  be  well  to  place  a  dam 
of  timber  at  the  outlet  of  the  bottom  culvert,  in  order  to  keep 
it  filled  with  water,  to  prevent  the  injury  which  the  bottom 
of  it  might  receive  from  the  water  discharged  from  the  upper 
culverts. 

The  side  walls  which  retain  the  earth  at  the  entrance  to  the 
culverts  should  be  arranged  with  grooves  to  receive'  pieces 
of  scantling  laid  horizontally  between  the  walls,  termed  stop- 
'planks^  to  form  a  temporary  dam,  and  cut  off  the  water  of  the 
reservoir,  in  case  of  repairs  to  the  culverts,  or  to  the  face  of 
the  dam. 

The  valves  are  small  sliding  gates,  which  are  raised  and 
lowered  by  a  rack  and  pinion,  or  by  a  screw.  The  cross  sec- 
tion of  the  culvert  is  contracted  by  a  partition,  either  of  ma- 
sonry or  timber,  at  the  point  where  the  valve  is  placed. 

791.  Dams  of  masonry  are  water-tight  walls,  of  suitable 
forms  and  dimensions  to  prevent  filtration,  and  resist  the 
pressure  of  water  in  the  reservoir.  The  most  suitable  cross- 
sedion  is  that  of  a  trapezoid,  the  face  towards  the  water  being 


CANALS. 


479 


vertical,  and  the  exterior  face  inclined  with  a  suitable  batter 
to  give  the  wall  sufficient  stability.  The  wall  should  be  at 
least  four  feet  ihick  at  the  water  line,  to  prevent  filtration, 
and  this  thickness  may  be  increased  as  circumstances  may  seem 
to  require.  Buttresses  should  be  added  to  the  exterior  facing, 
to  give  the  wall  greater  stability. 

792.  Suitable  dispositions  should  be  made  to  relieve  the 
dam  from  all  surplus  water  during  wet  seasons.  For  this  pur- 
pose arrangements  should  be  made  for  cutting  off  the  sources 
of  supply  from  the  reservoir  ;  and  a  cut,  termed  a  waste-weir 
(Fig  242),  of  suitable  width  and  depth,  should  be  made  at  some 
point  along  the  top  of  the  dam,  and  be  faced  with  stone,  or 
wood,  to  give  an  outlet  to  the  water  over  the  dam.  In  high 
dams  the  total  fall  of  the  water  should  be  divided  into  several 
partial  falls,  by  dividing  the  exterior  surface  over  which  the 
water  runs  into  offsets.  To  break  the  shock  of  the  water  up- 
on the  horizontal  surface  of  the  offset,  it  should  be  kept  cov- 
ered with  a  sheet  of  water  retained  by  a  dam  placed  across 
its  outlet. 


Fig.  242 — Represents  a  section  of  a  waste-weir  divided  into  two  falls. 

A,  body  of  the  dam. 

a,  top  of  the  waste-weir. 

6,  pool,  formed  by  a  stop-plank  dam  at  c,  to  break  the  fall  of  the  water, 

d,  covering  of  loose  stone  to  break  the  fall  of  the  water  from  the  pool  above. 


793.  In  extensive  reservoirs,  in  which  a  large  surface  is  ex- 
posed to  the  action  of  the  winds,  waves  might  be  forced  over 
the  top  of  the  dam,  and  subject  it  to  danger  ;  in  such  cases 
the  precaution  should  be  taken  of  placing  a  parapet  wall  to- 
wards the  outer  edge  of  the  top  of  the  dam,  and  facing  the 
top  throughout  with  flat  stones  laid  in  mortar. 

794.  Lift  of  locks.  The  engineer  is  not  always  left  free 
to  select  between  the  two  systems — that  of  isolated  locks  and 
locks  in  flights ;  for  the  form  of  the  natural  surface  of  the 
ground  may  compel  him  to  adopt  a  flight  of  locks  at  certain 
points.  As  to  the  comparative  expense  of  the  two  methods, 
a  flight  is  in  most  cases  cheaper  than  the  same  number  of 
single  locks,  as  there  are  certain  parts  of  the  masonry  which 


480 


CIVIL  ENGINEERING. 


can  be  suppressed.  There  is  also  an  economy  in  the  suppres- 
sion of  the  small  gates,  which  are  not  needed  in  flights.  It  is, 
however,  more  difficult  to  secure  the  foundations  of  combined 
than  of  single  locks  from  the  effects  of  the  w^ater,  which  forces 
its  way  from  the  upper  to  the  lower  level  under  the  locks. 
Where  an  active  trade  is  carried  on,  a  double  flight  is  some- 
times arranged  ;  one  for  the  ascending,  the  other  for  the 
descending  boats.  In  this  case  the  water  which  Alls  one  flight 
may,  after  the  passage  of  the  boat,  be  partly  used  for  the  other, 
by  an  arrangement  of  valves  made  in  the  side  wall  separating 
the  locks. 

The  lift  of  locks  is  a  subject  of  importance,  both  as  regards 
the  consumption  of  water  for  the  navigation,  and  the  economy 
of  construction.  Locks  with  great  lifts,  as  may  be  seen  from 
the  remarks  on  the  passage  of  boats,  consume  more  water 
than  those  with  small  lifts.  They  require  also  more  care  in 
their  construction,  to  preserve  them  from  accidents,  owing  to 
the  great  pressure  of  water  against  their  sides.  The  expense 
of  construction  is  otherwise  in  their  favor ;  that  is,  the  ex- 

Eense  will  increase  with  the  total  number  of  locks,  the 
eight  to  be  ascended  being  the  same.  The  smallest  lifts  are 
seldom  less  than  five  feet,  and  the  greatest,  for  ordinary 
canals,  not  over  twelve  ;  medium  lifts  of  seven  or  eight  feet 
are  considered  the  best  under  every  point  of  view.  I'his  is  a 
point,  however,  which  cannot  be  settled  arbitrarily,  as  the 
nature  of  the  foundations,  the  materials  used,  the  embank- 
ments around  the  locks,  the  changes  in  the  direction  of  the 
canal,  caused  by  varying  the  lifts,  are  so  many  modifying 
causes,  which  should  be  carefully  weighed  before  adopting  a 
definite  plan. 

The  lifts  of  a  flight  should  be  the  same  throughout ;  but  in 
isolated  locks  the  lifts  may  vary  according  to  circumstances. 
If  the  supply  of  water  from  the  summit  level  requires  to  be 
economized  with  care,  the  lifts  of  locks  which  are  furnished 
from  it  may  be  less  than  those  lower  down. 

795.  Levels.  The  position  and  the  dimensions  of  the 
levels  must  be  mainly  determined  by  the  form  of  the  natural 
surface.  Those  points  are  naturally  chosen  to  pass  from  one 
level  to  another,  or  as  the  positions  for  the  locks,  where  there 
is  an  abrupt  change  in  the  surface. 

A  level,  by  a  suitable  modification  of  its  cross  section,  can 
be  made  as  short  as  may  be  deemed  desirable  ;  there  being 
but  one  point  to  be  attended  to  in  this,  which  is,  that  a  boat 
passing  between  the  two  locks,  at  the  ends  of  the  level,  will 
nave  time  to  enter  either  lock  before  it  can  ground,  on  the 


CANALS. 


481 


"I -I 


Fig.  243 — Represent!?  a  plan  M,  and  a  section  N,  through  the  axis  of  a  single  lock  laid  on  a  ba- 
ton foundation.— A,  lock-chamber.  B,  fore-bay.  C,  tail-bay.  a,  a,  chamber-walls.  6,  6, 
recesses  or  chambers  in  the  side  walls  for  upper  gates,  c,  c,  lower-gate  chambers,  d,  d,  lift 
wall  and  upper  mitre  sill.  «,  e,  lower  mitre  sill.  A,  A,  tail  walls,  o,  o,  head  walls,  m,  m, 
upper  wing,  or  return  walls,  n,  n,  lower  wing  walls.  D,  body  of  masonry,under  the  fore-bay. 

31 


482 


CrVTL  ENGINEERESTG. 


supposition  that  the  water  drawn  off  to  fill  the  lower  lock, 
while  the  boat  is  traversing  the  level,  will  just  reduce  the 
depth  to  the  draught  of  the  boat. 

796.  Locks.  A  lock  (Fig.  243)  may  be  divided  into  three 
distinct  parts  :  1st.  The  part  included  between  the  two  gates, 
which  is  termed  the  chamber.  2d.  The  part  above  the  upper 
gates,  termed  fore^  or  head-bay.  3d.  The  part  below  the 
lower  gates,  termed  the  aft.,  or  tail-bay. 

797.  The  lock  chamber  must  be  wide  enough  to  allow  an 
easy  ingress  and  egress  to  the  boats  commonly  used  on  the 
canal ;  a  surplus  width  of  one  foot  over  the  width  of  the  boat 
across  the  beam  is  usually  deemed  sufficient  for  this  purpose. 
The  length  of  the  chamber  should  be  also  regulated  by  that 
of  the  boats ;  it  should  be  such,  that  when  the  boat  enters  the 
lock  from  the  lower  level,  the  tail-gates  may  be  shut  without 
requiring  the  boat  to  unship  its  rudder. 

The  plan  of  the  chamber  is  usually  rectangular,  as  this  form 
is,  in  every  respect,  superior  to  all  others.  In  the  cross  section 
of  the  chamber  (Fig.  24.4)  the  sides  receive  generally  a  slight 


Pig.  244— Represents  a  section  of  Fig.  243,  throxigh  the 

chamber. 

A,  A,  chamber  walls. 

B,  chamber  formed  with  an  inverted-arch  bottom. 

\ 

batter ;  as  when  so  arranged  they  are  found  to  give  greater  fa- 
cility to  the  passage  of  the  boat  than  when  vertical.  The  bot- 
tom of  the  chamber  is  either  flat  or  curved ;  more  water  will 
be  required  to  fill  the  flat-bottomed  chamber  than  the  curved, 
but  it  will  require  less  masonry  in  its  construction. 

798.  The  chamber  is  terminated  just  within  the  head  gates 
by  a  vertical  wall,  the  plan  of  which  is  usually  curved.  As 
this  wall  separates  the  upper  from  the  lower  level,  it  is 
termed  the  lift-wall ;  it  is  usually  of  the  same  height  as  the 
lift  of  the  levels.  The  top  of  the  lift-wall  is  formed  of  cut 
stone,  the  vertical  joints  of  which  are  normal  to  the  curved 
face  of  the  wall ;  this  top  course  projects  from  six  to  nine 
inches  above  the  bottom  of  the  upper  level,  presenting  an 
angular  point,  for  the  bottom  of  the  head-gates,  when  shut, 
to  rest  against.  This  is  termed  the  mitre-sill.  Various  de- 
grees of  opening  have  been  given  to  the  angle  between  the 
two  branches  of  the  mitre-sill  \  it  is,  however,  generally  so 


CANALS. 


483 


^  determined,  that  the  perpendicular  of  the  isosceles  triangle, 
formed  by  the  two  branches,  shall  vary  between  one-fifth  and 
one  sixth  of  tlie  base. 

As  stone  mitre-sills  are  liable  to  injury  from  the  shock  of 
the  gate,  they  are  now  usually  constructed  of  timber  (Fig.  245), 


Fig.  245 — Represents  a  plan  of  a  wooden  mitre-sill, 
and  a  horizontal  section  of  a  lock-gate  (Fig.  246) 
closed. 

a,  a,  mitre-sill  framed  with  the  pieces  6  and  c,  and 
firmly  fastened  to  the  side  walls  A,  A. 

d,  section  of  quoin  posts  of  lock-gate. 

e,  section  of  mitre  posts. 


by  framing  two  strong  beams  with  the  proper  angle  for  the 
gate  when  closed,  and  securing  them  firmly  upon  the  top  of 
the  lift-wall.  It  will  be  well  to  place  the  top  of  the  mitre- 
sill  on  the  lift-wall  a  little  lower  than  the  bottom  of  the 
canal,  to  preserve  it  from  being  struck  by  the  keel  of  the  boat 
on  entering  or  leaving  the  lock. 

799.  The  cross  section  of  the  chamber  walls  is  usually 
trapezoidal ;  the  facing  receives  a  slight  batter.  The  cham- 
ber walls  are  exposed  to  two  opposite  efforts ;  the  water  in 
the  lock  on  one  side,  and  the  embankment  against  the  w^all 
on  the  other.  The  pressure  of  the  embankment  is  the  greater 
as  well  as  the  more  permanent  effort  of  the  two.  The  di- 
mensions of  the  wall  must  be  regulated  by  this  pressure. 
The  usual  manner  of  doing  this,  is  to  make  the  wall  four  feet 
thick  at  the  water  line  of  the  upper  level,  to  secure  it  against 
filtration ;  and  then  to  determine  the  base  of  the  batter,  so 
that  the  mass  of  masonry  shall  present  sufficient  stability  to 
counteract  the  tendency  of  the  pressure.  The  spread,  and 
other  dimensions  of  the  foundations,  will  be  regulated  accord- 
ing to  the  nature  of  the  soil,  in  the  same  way  as  in  other 
structures. 

800.  The  bottom  of  the  chamber,  as  has  been  stated,  may 
be  either  flat  or  curved.  The  flat  bottom  is  suitable  to  very 
firm  soils,  which  will  neither  yield  to  the  vertical  pressure  of 
the  chamber  walls,  nor  admit  the  water  to  Alter  from  the 
upper  level  under  the  bottom  of  the  lock.  In  either  of  the 
contrary  cases,  the  bottom  should  be  made  with  an  inverted 
arch,  as  this  form  will  oppose  greater  resistance  to  the  up- 
ward pressure  of  the  water  under  the  bottom,  and  will  serve 
to  distribute  the  weight  of  the  walls  over  the  portion  of  the 
foundation  under  the  arch.    The  thickness  of  the  masonry  of 


484: 


CIVIL  ENGrNEERING. 


the  bottom  will  depend  on  the  width  of  the  chamber  and 
the  nature  of  the  soil.  Were  the  soil  a  solid  rock,  no  bottom- 
ing would  be  requisite ;  if  it  is  of  soft  mud,  a  very  solid  bot- 
toming, from  three  to  six  feet  in  thickness,  might  be  re- 
quisite. 

801.  The  principal  danger  to  the  foundations  arises  from 
the  water  which  may  filter  from  the  upper  to  the  lower  level, 
under  the  bottom  of  the  lock.  One  preventive  for  this,  but 
not  an  effectual  one,  is  to  drive  sheeting  piles  across  the  canal 
at  the  end  of  the  head-bay ;  another,  which  is  more  expensive, 
but  more  certain  in  its  effects,  consists  in  forming  a  deep 
trench  of  two  or  three  feet  in  width,  just  under  the  head-bay, 
and  filling  it  with  beton,  which  unites  at  the  top  with  the 
masonry  of  the  head-bay.  Similar  trenches  might  be  placed 
under  the  chamber  were  it  considered  necessary. 

802.  The  lift-wall  usually  receives  the  same  thickness  as 
the  chamber  walls ;  but,  unless  the  soil  is  very  firm,  it  would 
be  more  prudent  to  form  a  general  mass  of  masonry  under 
the  entire  head-bay,  to  a  level  with  the  base  of  the  chamber 
foundations,  of  which  mass  the  lift-wall  should  form  a  part. 

803.  The  head-hay  is  enclosed  between  two  parallel  walls, 
which  form  a  part  of  the  side  walls  of  the  lock.  They  are 
terminated  by  two  wing  walls,  which  it  will  be  found  most 
economical  to  run  back  at  right  angles  with  the  side  walls. 
A  recess,  termed  the  gate-chainber^  is  made  in  the  wall  of  the 
head-bay ;  the  depth  of  this  recess  should  be  sufiicient  to 
allow  the  gate,  when  open,  to  fall  two  or  three  inches  within 
the  facing  of  the  wall,  so  that  it  may  be  out  of  the  way  when 
a  boat  is  passing ;  the  length  of  the  recess  should  be  a  few 
inches  more  tlian  the  width  of  the  gate.  That  part  of  the 
recess  where  the  gate  turns  on  its  pivot  is  termed  the  hollow 
quoin  /  it  receives  what  is  termed  the  heel,  or  quoin-post  of 
the  gate,  wliich  is  made  of  a  suitable  form  to  fit  the  hollow 
quoin.  The  distance  between  the  hollow  quoins  and  the  face 
of  the  lift-wall  will  depend  on  the  pressure  against  the  mitre- 
sill,  and  the  strength  of  the  stone,  eighteen  inches,  will  gener- 
ally be  found  amply  sufiicient. 

The  side  walls  need  not  extend  more  than  twelve  inches 
beyond  the  other  end  of  the  gate-chamber.  The  wing  walls 
may  be  extended  back  to  the  total  width  of  the  canal,  but  it 
will  be  more  economical  to  narrow  the  canal  near  the  lock, 
and  to  extend  the  wing  walls  only  about  two  feet  into  the 
banks,  or  sides.  The  dimensions  of  the  side  and  wing  walls 
of  the  head-bay  are  regulated  in  the  same  way  as  the  cham- 
ber walls. 


CANALS. 


m 


The  bottom  of  the  head-bay  is  flat,  and  on  the  same  level 
with  the  bottom  of  the  canal ;  the  exterior  course  of  stones 
at  the  entrance  to  the  lock  should  be  so  jointed  as  not  to 
work  loose. 

804.  The  gate-chamhers  for  the  lower  gates  are  made 
in  the  chamber  walls ;  and  it  is  to  be  observed,  that  the  bot- 
tom of  the  chamber,  where  the  gates  swing  back,  should  be 
flat,  or  be  otherwise  arranged  not  to  impede  the  play  of  the 
gates. 

805.  The  side  walls  of  the  tail-hay  are  also  a  part  of 
the  general  side  walls,  and  their  thickness  is  regulated  as  in 
the  preceding  cases.  Their  length  will  depend  chiefly  on 
the  pressure  which  the  lower  gates  throw  against  them  when 
the  lock  is  full ;  and  partly  on  the  space  required  by  the 
lock-men  in  opening  and  shutting  gates  manoeuvred  by  the 
balance  beam.  A  calculation  must  be  made  for  each  par- 
ticular case,  to  ascertain  the  most  suitable  length.  The  side 
walls  are  also  terminated  by  wing  walls,  similarly  arranged 
to  those  of  the  head-bay.  The  points  of  junction  between 
the  wing  and  side  walls  should,  in  both  cases,  either  be 
curved,  or  the  stones  at  the  angles  be  rounded  off.  One  or 
two  perpendicular  grooves  are  sometimes  made  in  the  side 
walls  of  the  tail-bay,  to  receive  stop-planks,  when  a  tempo- 
rary dam  is  needed,  to  shut  off  the  water  of  the  lower  level 
from  the  chamber,  in  case  of  repairs,  etc.  Similar  arrange- 
ments might  be  made  at  the  head-bay,  but  they  are  not  indis- 
pensable in  either  case. 

The  strain  on  the  walls  at  the  hollow  quoins  is  greater 
than  at  any  other  points,  owing  to  the  pressure  at  those 
points  from  the  gates,  when  they  are  shut,  and  to  the  action 
of  the  gates  when  in  motion ;  to  counteract  this,  and 
strengthen  the  walls,  buttresses  should  be  placed  at  the  back 
of  the  walls  in  the  most  favorable  position  behind  the  quoins 
to  subserve  the  object  in  view. 

The  bottom  of  the  tail-bay  is  arranged,  in  all  respects,  like 
that  of  the  head-bay. 

806.  The  top  of  the  side  walls  of  the  lock  may  be  from 
one  to  two  feet  above  the  general  level  of  the  water  in  the 
upper  reach ;  the  top  course  of  the  masonry  being  of  heavy 
large  blocks  of  cut  stone,  although  this  kind  of  coping  is  not 
indispensable,  as  smaller  masses  have  been  found  to  suit  the 
same  purpose,  but  they  are  less  durable.  As  to  the  masonry 
of  the  lock  in  general,  it  is  only  necessary  to  observe  that 
those  parts  alone  need  be  of  cut  stone  where  there  is  great 
"wear  and  tear  from  any  cause,  as  at  the  angles  generally  ;  or 


m 


CrVTL  ETOINEERING. 


where  an  accurate  finish  is  indispensable,  as  at  the  hollow 
quoins.  The  other  parts  may  be  of  brick,  rubble,  beton,  etc., 
but  every  part  should  be  laid  in  the  best  hydraulic  mortar. 

807.  The  filling  and  emptying  the  lock  chamber  have 
given  rise  to  various  discussions  and  experiments,  all  of  which 
have  been  reduced  to  the  comparative  advantages  of  letting 
the  water  in  and  off  by  valves  made  in  the  gates  themselves, 
or  by  culverts  in  the  side  walls,  which  are  opened  and  shut 
by  valves.  "When  the  water  is  let  in  through  valves  in  the 
gates,  its  effects  on  the  sides  and  bottom  of  the  (chamber  are 
found  to  be  very  injurious,  particularly  in  high  lift-walls; 
besides  the  inconvenience  resulting  from  the  agitation  of  the 
boat  in  the  lock.  To  obviate  this,  in  some  degree,  it  has  been 
proposed  to  give  the  lift-wall  the  form  of  an  inclined  curved 
surface,  along  which  the  water  might  descend  without  pro- 
ducing a  shock  on  the  bottom. 

808.  The  side  culverts  are  small  arched  conduits,  of  a 
circular  or  an  elliptical  cross  section,  which  are  made  in  the 
mass  of  masonry  of  the  side  walls,  to  convey  the  water  from 
the  upper  level  to  the  chamber.  These  culverts,  in  some 
cases,  run  the  entire  length  of  the  side  walls,  on  a  level  with 
the  bottom  of  the  chamber,  from  the  lift-wall  to  the  end  of 
the  tail-wall,  and  have  several  outlets  leading  to  the  chamber. 
They  are  arranged  with  two  valves,  one  to  close  the  mouth 
of  the  culvert,  at  the  upper  level,  the  other  to  close  the  out- 
let from  the  chamber,  to  the  lower  level.  This  is,  perhaps, 
one  of  the  best  arrangements  for  side  culverts.  They  all 
present  the  same  difficulty  in  making  repairs  when  out  of 
order,  and  they  are  moreover  very  subject  to  accidents. 
They  are  therefore  on  these  accounts  inferior  to  valves  in  the 
gates. 

809.  It  has  also  been  proposed,  to  avoid  the  inconveniences 
of  culverts,  and  the  disadvantages  of  lift- walls,  by  suppress- 
ing the  latter,  and  gradually  increasing  the  depth  of  the 
Tipper  level  to  the  bottom  of  the  chamber.  This  method 
presents  a  saving  in  the  mass  of  masonry,  but  the  gates  will 
cost  more,  as  the  head  and  tail  gates  must  be  of  the  same 
height.  It  would  entirely  remove  the  objection  to  valves  in 
the  gates,  as  the  current  through  them,  in  this  case,  would 
not  be  sufficiently  strong  to  injure  the  masonry. 

810.  The  hottom  of  the  canal  below  the  lock  should  be  pro- 
tected b}^  what  is  termed  an  apron,  which  is  a  covering  of 
plank  laid  on  a  grillage,  or  else  one  of  brushwood  and  dry 
Btone.  The  sides  should  also  be  faced  with  timber  or  dry  stone. 
The  length  of  this  facing  will  depend  on  the  strength  of  the 


CANALS. 


m 


cnrrent ;  generally  not  more  than  from  fifteen  to  thirty  feet 
from  the  lock  will  require  it.  The  entrance  to  the  head-bay 
is,  in  some  cases,  similarly  protected,  but  this  is  unnecessary, 
as  the  current  has  but  a  very  slight  effect  at  that  point. 

811.  Locks  constructed  of  timber  and  dry  stone,  termed 
com.jposite-lochs^  are  to  be  met  with  on  several  of  the  canals  of 
the  United  States.  The  side  walls  are  formed  of  dry  stone 
carefully  laid  ;  the  sides  of  the  chamber  being  faced  with 
plank  nailed  to  horizontal  and  upright  timbers,  which  are  firm- 
ly secured  to  the  dry  stone  walls.  The  walls  rest  upon  a  plat- 
form laid  upon  heavy  beams  placed  transversely  to  the  axis 
of  the  lock.  The  bottom  of  the  chamber  usually  receives  a 
double  thickness  of  plank.  The  quoin-posts  and  mitre-sills 
are  formed  of  heavy  beams. 

812.  Lock  Gates.    A  lock  gate  (Fig.  246)  is  composed  of 


11 


Fig.  246— Represents 
the  elevation  of  a 
lock-gate  closed. 

o,  a,  qiioin-posts . 

&,  mitre- posts. 

c,  c,  cross  pieces 
framed  into  a  and 
6,  and  firmly  con- 
nected with  them 
by  wrought  -  iron 
plates. 

o,  plank  or  sheath- 
ing of  the  gate. 

d,  valve. 

fn,7n,  balance-beams. 


two  leaves,  each  leaf  consisting  of  a  solid  framework  covered 
on  the  side  towards  the  water  with  thick  plank  made  water- 
tight. The  frame  usually  consists  of  two  uprights,  of  several 
horizontal  cross  pieces  let  into  the  uprights,  and  sometimes  a 
diagonal  piece  or  brace,  intended  to  keep  the  frame  of  an  in- 
variable form,  is  added.  The  upright,  around  which  the  leaf 
turns,  termed  the  quoin  or  heel-j)ost,  is  rounded  off  on  the  back 
to  fit  in  the  hollow  quoin ;  it  is  made  slightly  eccentric  with  it, 
BO  that  it  may  turn  easily  without  rubbing  against  the  quoin ; 
its  lower  end  rests  on  an  iron  gudgeon,  to  which  it  is  fitted  by 
a  corresponding  indentation  in  an  iron  socket  on  the  end ;  the 
upper  extremity  is  secured  to  the  side  walls  by  an  iron  collar, 
within  which  the  post  turns.  The  collar  is  so  arranged  that 
it  can  be  easily  fastened  to,  or  loosened  from,  two  iron  bars, 


488 


CmL  ENGINEERING. 


termed  anchor-irons,  which  are  firmly  attached  by  bolts,  or  a 
lead  sealing,  to  the  top  course  of  the  walls.  One  oi  the  anchor- 
irons  is  placed  in  a  line  with  the  leaf  when  shut,  the  other  in 
a  line  with  it  when  open,  to  resist  most  effectually  the  strain 
in  those  two  positions  of  the  gate.  The  opposite  upright, 
termed  the  mitre-jpost,  has  one  edge  bevelled  off  to  fit  against 
the  mitre-post  of  the  other  leaf  of  the  gate. 

813.  A  long  heavy  beam,  termed  a  oalance-heam,  from  its 
partially  balancing  the  weight  of  the  leaf,  rests  on  the  quoin- 
post,  to  which  it  is  secured,  and  is  mortised  with  the  mitre- 
post.  The  balance-beam  should  be  about  four  feet  above  the 
top  of  the  lock,  to  be  readily  manoeuvred ;  its  principal  use 
being  to  open  and  shut  the  leaf. 

814.  The  top  cross  piece  of  the  gate  should  be  about  on  a 
level  with  the  top  of  the  lock ;  the  bottom  cross  piece  should 
swing  clear  of  the  bottom  of  the  lock.  The  position  of  the 
intermediate  cross  pieces  may  be  made  to  depend  on  their 
dimensions  :  if  they  are  of  the  same  dimensions,  they  should 
be  placed  nearer  together  at  the  bottom,  as  the  pressure  of  the 
water  is  there  greatest ;  but,  by  making  them  of  unequal  di- 
mensions, they  may  be  placed  at  equal  distances  apart ;  this, 
however,  is  not  of  much  importance  except  for  large  gates, 
and  considerable  depths  of  water. 

The  plank  may  be  arranged  either  parallel  to  the  uprights, 
or  parallel  to  the  diagonal  brace  ;  in  the  latter  position  they 
will  act  with  the  brace  to  preserve  the  form  of  the  frame. 

815.  A  wide  board,  supported  on  brackets,  is  often  affixed 
to  the  gates,  both  for  the  manoeuvre  of  the  machinery  of  the 
valves,  and  to  serve  as  a  foot-bridge  across  the  lock.  The 
valves  are  small  gates  which  are  arranged  to  close  the  open- 
ings made  in  the  gates  for  letting  in  or  drawing  off  the  water. 
They  are  arranged  to  slide  up  and  down  in  grooves,  by  the 
aid  of  a  rack  and  pinion,  or  a  square  screw ;  or  they  may  be 
made  to  open  or  shut  by  turning  on  a  vertical  axis,  in  which 
case  they  are  termed  paddle  gates.  The  openings  in  the  up- 
per gates  are  made  between  the  two  lowest  cross  pieces,  in 
the  lower  gates  the  openings  are  placed  just  below  the  surface 
of  the  water  in  the  reach.  The  size  of  the  opening  will 
depend  on  the  time  in  which  it  is  required  to  fill  the  lock. 

816.  Accessory  Works.  Under  this  head  are  classed  those 
constructions  which  are  not  a  part  of  the  canal  proper,  although 
generally  found  necessary  on  all  canals :  as  the  culverts  for 
conveying  off  the  water-courses  which  intersect  the  line  of  the 
canal ;  the  inlets  of  feeders  for  the  supply  of  water ;  aqueduct 
bridges,  etc.,  etc. 


CANALS. 


489 


817.  Culverts.  The  disposition  to  be  made  of  water-courses 
intersecting  the  line  of  the  canal  will  depend  on  their  size, 
the  character  of  their  current,  and  the  relative  positions  of 
the  canal  and  stream. 

Small  brooks  which  lie  lower  than  the  canal  may  be  con- 
veyed under  it  through  an  ordinary  culvert.  If  the  level  of 
the  canal  and  brook  is  nearly  the  same,  it  will  then  be  neces- 
sary to  make  the  culvert  in  the  shape  of  an  inverted  syphon, 
and  it  is  therefore  termed  a  broken-hack  culvert.  If  the 
water  of  the  brook  is  generally  limpid,  and  its  current  gentle, 
it  may,  in  the  last  case,  be  received  into  the  canal.  The 
communication  of  the  brook,  or  feeder,  with  the  canal,  should 
be  so  arranged  that  the  water  may  be  shut  off,  or  let  in  at 
pleasure,  in  any  quantity  desired.  For  this  purpose  a  cut  is 
made  through  the  side  of  the  canal,  and  the  sides  and  bottom 
of  the  cut  are  faced  with  masonry  laid  in  hydraulic  mortar. 
A  sliding  gate,  fitted  into  two  grooves  made  in  the  side  walls, 
is  manoeuvred  by  a  rack  and  pinion,  so  as  to  regulate  the 
quantity  of  water  to  be  let  in.  The  water  of  the  feeder,  or 
brook,  should  first  be  received  in  a  basin,  or  reservoir,  near 
the  canal,  where  it  may  deposit  its  sediment  before  it  is  drawn 
off.  In  cases  where  the  line  of  the  canal  is  crossed  by  a  tor- 
rent, which  brings  down  a  large  quantity  of  sand,  pebbles, 
etc.,  it  may  be  necessary  to  make  a  permanent  structure  over 
the  canal,  forming  a  channel  for  the  torrent ;  but  if  the  dis- 
charge of  the  torrent  is  only  periodical,  a  movable  channel 
may  be  arranged,  for  the  same  purpose,  by  constructing  a 
boat  with  a  deck  and  sides  to  form  the  water-way  of  the  tor- 
rent. The  boat  is  kept  in  a  recess  in  the  canal  near  the  point 
where  it  is  used,  and  is  floated  to  its  position,  and  sunk  when 
wanted. 

818.  Aqueducts,  etc.  When  the  line  of  the  canal  is  inter- 
sected by  a  wide  water-course,  the  communication  between 
the  two  shores  must  be  effected  either  by  a  canal  aqueduct 
bridge,  or  by  the  boats  descending  from  the  canal  into  the 
stream.  As  the  construction  of  aqueduct  bridges  has  already 
been  considered,  nothing  farther  on  this  point  need  here  be 
added.  The  expedient  of  crossing  the  stream  by  the  boats 
may  be  attended  with  many  grave  inconveniences  in  water- 
courses liable  to  freshets,  or  to  considerable  variations  of  level 
at  different  seasons.  In  these  cases  locks  must  be  so  arranged 
on  each  side,  where  the  canal  enters  the  stream,  that  boats 
may  pass  from  the  one  to  the  other  under  all  circumstances 
of  difference  of  level  between  the  two.  The  locks  and  the 
portions  of  the  canal  which  join  the  stream  must  be  secured 


490 


CIVIL  ENGINEERING. 


against  damage  from  freshets  by  suitable  embanlanents ;  and, 
when  the  summer  water  of  the  stream*  is  so  low  that  the 
navigation  would  be  impeded,  a  dam  across  the  stream  will 
be  requisite  to  secure  an  adequate  depth  of  water  during  this 
epoch. 

819.  Canal-Bridges.  Bridges  for  roads  over  a  canal,  termed 

canal-hridges^  are  constructed  like  other  structures  of  the 
same  kind.  In  planning  them  the  engineer  should  endeavor 
to  give  sufficient  height  to  the  bridge  to  prevent  those  acci- 
dents, of  but  too  frequent  occurrence,  from  persons  standing 
upright  on  the  deck  of  the  passage-boat  while  passing  under 
a  bridge. 

A  novel  device,  which,  on  account  of  its  diminutive  size,  is 
hardly  w^orthy  of  the  name  of  a  bridge,  is  used  for  crossing 
the  canal  at  Williamsport,  Pennsylvania.  It  is  really  a  small 
pivot  bridge,  so  constructed  that  a  boat  may  push  it  open  either 
way  as  desired  as  it  passes  through,  and  which  will  close  itself 
after  the  boat  has  passed.  As  it  opens  it  moves  up  an  in- 
clined plane,  so  that  its  weight  will  aid  in  closing  it.  A 
weight,  which  is  attached  to  a  rope  at  one  end,  the  rope 
passing  over  a  pulley  and  attached  to  the  bridge  at  the  other, 
is  also  employed  in  closing  it. 

820.  Waste-Weir.  Waste-weirs  must  be  made  along  the 
levels  to  let  off  the  surplus  water.  The  best  position  for  them 
is  at  points  w^here  they  can  discharge  into  natural  water- 
courses. The  best  arrangement  for  a  waste-weir  is  to  make 
a  cut  through  the  side  of  the  canal  to  a  level  w^ith  the  bottom 
of  it,  so  that,  in  case  of  necessity,  the  waste-weir  may  also 
serve  for  draining  the  level.  The  sides  and  bottom  of  the  cut 
must  be  faced  with  masonry,  and  have  grooves  left  in  them 
to  receive  stop-plank,  or  a  sliding  gate,  over  which  the  sur- 
plus water  is  allowed  to  flow,  under  the  usual  circumstances, 
but  which  can  be  removed,  if  it  be  found  necessary,  either 
to  let  off  a  larger  amount  of  water,  or  to  drain  the  level 
completely. 

821.  Temporary  Dams.  In  long  levels  an  accident  hap- 
pening at  any  one  point  might  cause  serious  injury  to  the 
navigation,  besides  a  great  loss  of  water.  To  prevent  this,  in 
some  measure,  the  wddth  of  the  canal  may  be  diminished,  at 
several  points  of  a  long  level,  to  the  width  of  a  lock,  and  the 
sides,  at  these  points,  may  be  faced  with  masonry,  arranged 
with  grooves  and  stop-planks,  to  form  a  temporary  dam  for 
shutting  off  the  water  on  either  side. 

822.  Tide,  or  Guard  Lock.  The  point  at  which  a  canal 
enters  a  river  requires  to  be  selected  with  judgment.  Gen- 


CANALS. 


erally  speaking,  a  bar  will  be  found  in  the  principal  water- 
course at  or  below  the  points  where  it  receives  its  affluents. 
When  the  canal,  therefore,  follows  the  valley  of  an  affluent, 
its  outlet  should  be  placed  below  the  bar,  to  render  its  navi- 
gation permanently  secure  from  obstruction.  A  large  basin 
is  usually  formed  at  the  outlet,  for  the  convenience  of  com- 
merce ;  and  the  entrance  from  this  basin  to  the  canal,  or  from 
the  river  to  the  basin,  is  effected  by  means  of  a  lock  with 
double  gates,  so  arranged  that  a  boat  can  be  passed  either 
way,  according  as  the  level  in  the  one  is  higher  or  lower  than 
that  in  the  other.  A  lock  so  arranged  is  termed  a  tide  or 
guard  lock^  from  its  uses.  The  position  of  the  tail  of  this 
lock  is  not  indifferent  in  all  cases  where  it  forms  the  outlet  to 
the  river ;  for,  were  the  tail  placed  up  stream,  it  would  be 
more  difficult  to  pass  in  or  out  than  if  it  were  down  stream. 

823.  The  general  dimensions  of  canals  and  their  locks  in 
this  country  and  in  Europe,  with  occasional  exceptions,  do  not 
differ  in  any  considerable  degree. 

English  Canals.  Two  classes  of  canals  are  to  be  met 
with  in  England,  differing  materially  in  their  dimensions. 
The  following  are  the  usual  dimensions  of  the  cross  section 
of  the  largest  size,  and  those  of  their  locks :  — 

Width  of  section  at  the  water  level,  from  36  to  40  feet. 


Width  at  bottom                                           24  " 

Depth                                                         5  " 

Length  of  lock  between  mitre-sills              75  to  80  " 

Width  of  chamber                                         15  " 


The  Caledonian  canal,  in  Scotland,  which  connects  Loch 
Eil  on  the  Western  sea  with  Murray  Firth  on  the  Eastern,  is 
remarkable  for  its  size,  which  will  admit  of  the  passage  of 
frigates  of  the  second  class.  The  following  are  the  principal 
dimensions  of  the  cross  section  of  the  canal  and  its  locks : — 


Width  of  canal  at  the  water  level   110  feet. 

Width  at  bottom   50  " 

Depth  of  water   20  " 

Width  of  berm  ,     6  " 

Length  of  lock  between  mitre-sills   180  " 

Width  of  chamber  at  top..   40  " 

Lift  of  lock   8  " 


The  side  walls  of  the  locks  are  built  with  a  curved  batter, 
they  are  of  the  uniform  thickness  of  6  feet,  and  are  strength- 
ened by  counterforts,  placed  about  15  feet  apart,  which  are 
4  feet  wide  and  of  the  same  thickness.  The  bottom  of  the 
chamber  is  formed  with  an  inverted  arch. 


492 


CIVIL  ENGINEERING. 


French  Canals.    In  France  the  following  uniform  system 


has  been  established  for  the  dimensions  of  canals  and  their 
locks  : — 

Width  of  canal  at  water  level  52  feet. 

Width  at  bottom  33  to  36  " 

Depth  of  water   5  " 

Length  of  lock  between  mitre-sills  115  " 

Width  of  lock   IT  " 

The  boats  adapted  to  these  dimensions  are  from  105  to  108 
feet  long,  16^  feet  across  the  beam,  and  have  a  draught  of  4 
feet. 

Width  of  canal  at  top   50  feet. 

Width  at  bottom   30  " 

Depth  of  water   5  " 

Length  of  locks  100  " 

Width  of  locks   15  " 


The  Eideau  canal,  which  connects  Lake  Ontario  with  the 
Hiver  Ottawa,  is  arranged  for  steam  navigation.  A  consider- 
able portion  of  this  line  consists  of  slack-water  navigation, 
formed  by  connecting  the  natural  water-courses  between  the 
outlets  of  the  canal.  The  length  of  the  locks  on  this  canal  is 
134  feet  between  the  mitre-sills,  and  their  width  33  feet. 

The  Welland  canal,  between  lakes  Erie  and  Ontario,  as  ori- 
ginally constructed,  received  the  following  dimensions : — 


Width  of  canal  at  top   56  feet. 

Width  at  bottom   24  " 

Depth  of  water   ....    8  " 

Length  of  locks  between  mitre-sills  110  " 

Width  of  locks   22  " 


The  canals  and  locks  made  to  avoid  the  dangerous  rapids 
of  the  St.  Lawrence  are  in  all  respects  among  the  largest  in 
the  world.  The  following  are  the  dimensions  of  the  por- 
tion of  the  canal  and  the  locks  between  Long  Sault  and  Corn- 
wall : — 


Width  of  canal  at  top   132  feet. 

Width  at  bottom   100  « 

Depth  of  water   8  " 

Width  of  tow-path   12  " 

Length  of  locks  between  mitre-sills   200  " 

Width  of  locks  at  top   56.6  " 

Width  of  locks  at  bottom   43  " 


A  berm  of  5  feet  is  left  on  each  side  between  the  water- 


CANALS. 


493 


way  and  tlie  foot  of  the  interior  slope  of  the  tow-path.  The 
height  of  the  tow-path  is  6  feet  above  the  berm.  By  increas- 
ing the  depth  of  water  in  the  canal  to  10  feet,  the  water-line 
at  top  can  be  increased  to  150  feet. 

The  dimensions  of  the  Erie  canal  as  enlarged  are  : — 

Width  of  canal  at  top,  with  bench  walls.. . .  81  feet. 


Width  of  canal  at  top,  without  bench  walls.  75  " 

Width  of  canal  at  water  surface   YO  " 

Width  of  canal  at  bottom,  with  bench  walls.  42  " 
Width  of  canal  at  bottom,  without  bench 

walls   52i  " 

Depth  of  water   7  " 

Width  of  tow-path   14  " 

Width  of  locks  at  top   18  "  10  in. 

Width  of  locks  at  bottom   17  "  4i  in. 

Length  of  lock  (between  mitre-sills)  110  " 

824.  Locomotion  on  Canals.    In  early  times  boats  were 


drawn  or  pushed  along  by  servants  or  slaves.  In  civilized 
countries  horses  and  mules  have  been  chiefly  used.  A  few 
years  since  several  attempts  were  made  to  use  steam  power, 
by  driving  the  boat  like  a  propeller,  and  although  it  would 
do  the  work,  yet  it  was  mostly  abandoned  after  a  few  months. 
The  wheel  created  such  a  disturbance  in  the  water  as  caused 
it  to  wash  the  banks  and  thus  damage  them. 

A  system,  known  as  the  Belgian  system,  has  been  quite 
extensively  used  in  some  of  the  European  countries.  It  con- 
sists of  a  cable  which  passes  from  one  end  of  the  canal  to  the 
other,  and  is  sunk  in  it.  It  is  wound  around  a  wheel  which 
is  at  one  end  of  the  boat.  Steam  power  is  applied  to  turn 
the  wheel,  and,  as  the  friction  of  the  rope  on  the  wheel  pre- 
vents it  from  slipping,  it  will  take  up  the  cable  on  one  side  of 
the  wheel  and  let  it  out  on  the  other,  and  thus  draw  the  boat 
along.  One  of  the  objections  to  this  plan  is,  it  requires  a 
large  amount  of  slack  cable  to  accommodate  a  large  traffic, 
and  every  boat  must  draw  in  all  the  slack  every  time  it  passes 
over  the  canal. 

During  the  winter  of  1870-71  the  Legislature  of  the  State 
of  ]^ew  York  offered  a  prize  of  $100,000  to  the  party  who 
would  make  an  acceptable  mode  of  applying  steam  for  pro- 
pelling canal  boats  on  the  canals,  and  no  plan  was  to  be  con- 
sidered which  involved  the  Belgian  system.  The  engineer  in 
charge  of  this  project  states  that  in  round  numbers  a  thousand 
lans,  coming  from  all  parts  of  the  world,  have  been  presented^ 
ut  up  to  the  present  time  the  prize  has  not  been  awarded. 


494 


OIVIL  ENGLNEERINa. 


CHAPTEK  IX. 

EIVEKS. 

825.  Natural  features  of  Bivers.  All  rivers  present  the 
same  natural  features  and  phenomena,  which  are  more  or  less 
strongly  marked  and  diversified  by  the  character  of  the  re- 
gion through  which  they  flow.  Taking  their  rise  in  the  high- 
lands, and  gradually  descending  thence  to  some  lake,  or  sea, 
their  beds  are  modified  by  the  nature  of  the  soil  of  the  val- 
leys in  which  they  lie,  and  the  velocities  of  their  currents  are 
affected  by  the  same  cause.  Near  their  sources,  their  beds 
are  usually  rocky,  irregular,  narrow,  and  steep,  and  their 
currents  are  rapid.  Approaching  their  outlets,  the  beds  be- 
come wider  and  more  regular,  the  declivity  less,  and  the  cur- 
rent more  gentle  and  uniform.  In  the  upper  portions  of  the 
beds,  their  direction  is  more  direct,  and  the  obstructions  met 
with  are  usually  of  a  permanent  character,  arising  from  the 
inequalities  of  the  bottom.  In  the  lower  portions,  the  beds 
assume  a  more  tortuous  course,  win  din  throus^h  their  val- 
leys,  and  forming  those  abrupt  bends,  termed  elbows^  which 
seem  subject  to  no  fixed  laws ;  and  here  are  found  those  ob- 
structions, of  a  more  changeable  character,  termed  bars, 
which  are  caused  by  deposites  in  the  bed,  arising  from  the 
wear  of  the  banks  by  the  current. 

826.  The  relations  which  are  found  to  exist  between  the 
cross  section  of  a  river,  its  longitudinal  slope,  the  nature  of 
its  bed,  and  its  volume  of  water,  are  termed  the  regimen  of 
the  river.  When  these  relations  remain  permanently  invari- 
able, or  change  insensibly  with  time,  the  river  is  said  to  have 
a  fixed  regimen. 

Most  rivers  acquire  in  time  a  fixed  regimen,  although  peri- 
odically, and  sometimes  accidentally,  subject  to  changes  from 
freshets  caused  by  the  melting  of  snow,  and  heavy  falls  of 
rain.  These  variations  in  the  volume  of  water  thrown  into 
the  bed  cause  corresponding  changes  in  the  velocity  of  the 
current,  and  in  the  form  and  dimensions  of  the  bed.  These 
changes  will  depend  on  the  character  of  the  soil,  and  the 
width  of  the  valley.  In  narrow  valleys,  where  the  banks  do 
not  readily  yield  to  the  action  of  the  current,  the  effects  of 


BIVEE8. 


any  variation  of  velocity  will  only  be  temporarily  to  deepen 
the  bed.  In  wide  valleys,  where  the  soil  of  the  banks  is 
more  easily  worn  by  the  current  than  the  bottom,  any  in- 
crease in  the  volume  of  water  will  widen  the  bed ;  and  if 
one  bank  yields,  more  than  the  other,  an  elbow  will  be 
formed,  and  the  position  of  the  bed  will  be  gradually  shifted 
towards  the  concave  side  of  the  elbow. 

827.  The  formation  of  elbows  occasions  also  variations  in 
the  deptli  and  velocity  of  the  water.  The  greatest  depth  is 
found  at  the  concave  side.  At  the  straight  portions  which 
connect  two  elbows,  the  depth  is  found  to  decrease,  and  the 
velocity  of  the  current  to  increase.  The  bottom  of  the  bed 
thus  presents  a  series  of  undulations,  forming  shallows  and 
deep  pools,  with  rapid  and  gentle  currents. 

828.  Bars  are  formed  at  those  points,  where  from  any 
cause  the  velocity  of  the  current  receives  a  sudden  check. 
The  particles  suspended  in  the  water,  or  borne  along  over  the 
bottom  of  the  bed  by  the  current,  are  deposited  at  these 
points,  and  continue  to  accumulate,  until,  by  the  gradual  fil- 
ling of  the  bed,  the  water  acquires  sufficient  velocity  to  bear 
farther  on  the  particles  that  reach  the  bar,  when  the  river  at 
this  point  acquires  and  retains  a  fixed  regimen,  until  dis- 
turbed by  some  new  cause. 

829.  The  points  at  which  these  changes  of  velocity  usually  * 
take  place,  and  near  which  bars  are  found,  are  at  the  junc- 
tion of  a  river  with  its  affluents,  at  those  points  where  the 
bed  of  the  river  receives  a  considerable  increase  in  width,  at 
the  straight  portions  of  the  bed  between  elbows,  and  at  the 
outlet  of  the  river  to  the  sea.  The  character  of  the  bars  will 
depend  upon  that  of  the  soil  of  the  banks,  and  the  velocity 
of  the  current.  Generally  speaking,  the  bars  in  the  upper 
portions  of  the  bed  will  be  composed  of  particles  which  are 
larger  than  those  by  which  they  are  formed  lower  down. 
These  accumulations  at  the  mouths  of  large  rivers  form  in 
time  extensive  shallows,  and  great  obstructions  to  the  dis- 
charge of  the  water  during  the  seasons  of  freshets.  The 
river  then,  not  finding  a  sufficient  outlet  by  the  ordinary 
channel,  excavates  for  itself  others  through  the  most  yielding 
parts  of  the  deposites.  In  this  manner  are  formed  those 
features  which  characterize  the  outlets  of  many  large  rivers, 
and  which  are  termed  delta,  after  the  name  given  to  the  pe- 
culiar shape  of  the  outlets  of  the  Nile. 

830.  River  Improvements.  There  is  no  subject  that 
falls  within  the  province  of  the  engineer's  art,  that  presents 
greater  difficulties  and  more  uncertain  issues  than  the  im- 


496 


CIVIL  ENGINEERING. 


provement  of  rivers.  Ever  subject  to  important  changes  in 
their  regimen,  as  the  regions  by  which  they  are  fed  are 
cleared  of  their  forests  and  brought  under  cultivation,  one 
century  sees  them  deep,  flowing  with  an  equable  current,  and 
liable  only  to  a  gradual  increase  in  volume  during  the  sea- 
sons of  freshets ;  while  the  next  finds  their  beds  a  prey  to 
sudden  and  great  freshets,  which  leave  them,  after  their  vio- 
lent passage,  obstructed  by  ever  shifting  bars  and  elbows. 
Besides  these  revolutions  brought  about  in  the  course  of 
years,  every  obstruction  temporarily  placed  in  the  way  of  the 
current,  every  attempt  to  guard  one  point  from  its  action  by 
any  artificial  means,  inevitably  produces  some  corresponding 
change  at  another,  which  can  seldom  be  foreseen,  and  for 
which  the  remedy  applied  may  prove  but  a  new  cause  of 
harm.  Thus,  a  bar  removed  from  one  point  is  found  gradu- 
ally to  form  lower  down ;  one  bank  protected  from  the  cur- 
rent's force  transfers  its  action  to  the  opposite  one,  on  any 
increase  of  volume  from  freshets,  widening  the  bed,  and 
frequently  giving  a  new  direction  to  the  channel.  Owing 
to  these  ever  varying  causes  of  change,  the  best  weighed 
plans  of  river  improvement  sometimes  result  in  complete 
failure. 

831.  In  forming  a  plan  for  a  river  improvement,  the 
principal  objects  to  be  considered  by  the  engineer,  are,  1st. 
The  means  to  be  taken  to  protect  the  banks  from  the  action 
of  the  current.  2d.  Those  to  prevent  inundations  of  the  sur- 
rounding country.  3d.  The  removal  of  bars,  elbows  and  other 
natural  obstructions  to  navigation.  4th.  The  means  to  be  re- 
sorted to  for  obtaining  a  suitable  depth  of  water  for  boats,  of 
a  proper  tonnage,  for  the  trade  on  the  river. 

832.  Means  for  protecting  the  banks.  To  protect  the 
banks,  either  the  velocity  of  the  current  in-shore  must  be  de- 
creased so  as  to  lessen  its  action  on  the  soil ;  or  else  a  facing 
of  some  material  sufficiently  durable  to  resist  its  action  must 
be  employed.  The  former  method  may  be  used  when  the 
banks  are  low  and  have  a  gentle  declivity.  The  simplest  plan 
for  this  purpose  consists  either  in  planting  such  shrubbery  on 
the  declivity  as  will  tln-ive  near  water;  or  by  driving  down 
short  pickets  and  interlacing  them  with  twigs,  forming  a  kind 
of  wicker-work.  These  constructions  break  the  force  of  the 
current,  and  diminish  its  velocity  near  the  shore,  and  thus 
cause  the  water  to  deposit  its  finer  particles,  which  gradually 
fill  out  and  strengthen  the  banks.  If  the  banks  are  high,  and 
are  subject  to  cave  in  from  the  action  of  the  current  on  their 
base,  they  may  be  either  cut  down  to  a  gentle  declivity,  as  in 


EIVERS. 


497 


the  last  case ;  or  else  they  may  receive  a  slope  of  nearly  45^, 
and  be  faced  with  dry  stone,  care  being  taken  to  secure  the 
base  by  blocks  of  loose  stone,  or  by  a  facing  of  brush  and  stone 
laid  in  alternate  layers. 

833.  Measures  against  inundations.  At  the  points  in 
the  course  of  a  river  where  inundations  are  to  be  apprehend- 
ed, the  water-way,  if  practicable,  should  be  increased ;  all 
obstructions  to  the  free  discharge  of  the  water  below  the  point 
should  be  removed ;  and  dikes  of  earth,  usually  termed  levees^ 
should  be  raised  on  each  side  of  the  river.  By  increasing  the 
water-way  a  temporary  improvement  only  will  be  effected  ^ 
for,  except  in  the  season  of  freshets,  the  velocity  of  the  cur- 
rent at  this  point  will  be  so  much  decreased  as  to  form  de- 
posites,  which,  at  some  future  day,  may  prove  a  cause  of 
damage.  In  confining  the  water  between  levees,  two  methods 
have  been  tried  :  the  one  consists  in  leaving  a  water-way  strict- 
ly necessary  for  the  discharge  of  freshets ;  the  other  in  giving 
the  stream  a  wide  bed.  The  Po  in  Italy  and  the  Mississippi 
present  examples  of  the  former  method  ;  the  effect  of  which 
in  both  cases  has  been  to  raise  the  bed  of  the  stream  so  much 
that  in  many  parts  the  water  is  liabitually  above  the  natural 
surface  of  the  country,  leaving  it  exposed  to  serious  inunda-^ 
tions  should  the  levees  give  way.  The  other  method  has  been' 
tried  on  the  Loire  in  France,  and  observation  has  proved  that' 
the  general  level  of  the  bed  has  not  sensibly  risen  for  a  long 
series  of  years  ;  but  it  has  been  found  that  the  bars,  which  are 
formed  after  each  freshet,  are  shifted  constantly  by  the  next,' 
so  that  when  the  waters  have  subsided  to  their  ordinary  state^ 
the  navigation  is  extremely  intricate  from  this  cause.  Other 
means  have  been  tried,  such  as  opening  new  channels  at  the 
exposed  points,  or  building  dams  above  them  to  keep  the 
water  back ;  but  they  have  all  been  found  to  afford  only  a  tem- 
porary relief. 

834.  Elbows.  The  constant  wear  of  the  bank,  and  shift- 
ing of  the  channel  towards  the  concave  side  of  elbows,  have 
led  to  A'arious  plans  for  removing  the  inconveniences  which 
they  present  to  navigation.  The  method  which  has  been 
most  generally  tried  for  this  purpose  consists  in  building  out 
dikes,  termed  wing-dams^  from  the  concave  side  into  the 
stream,  placing  them  either  at  right  angles  to  the  thread  of 
the  current,  or  obliquely  down  stream,  so  as  to  deflect  the  cur- 
rent towards  the  opposite  shore. 

Wing-dams  are  usually  constructed  either  of  blocks  of 
stone,  of  crib-work  formed  of  heavy  timbers  filled  in  with 
jbroken  stone,  or  of  alternate  layers  of  gravel  and  fascines. 
32 


498 


CTVIL  ENGINEERING. 


Within  a  few  years  back,  wing-dams,  consisting  simply  of  a 
series  of  vertical  frames,  or  ribs  (Fig.  247),  strongly  con- 
nected together,  and  covered  on  the  up-stream  side  by  thick 
plank,  which  present  a  broken  inclined  plane  to  the  current, 
the  lower  part  of  which  is  less  steep  than  the  upper,  have 
been  used  upon  the  Po,  with,  it  is  stated,  complete  success, 
for  arresting  the  wear  of  a  bank  by  the  current.  These 
dams  are  placed  at  some  distance  above  the  point  to  be  pro- 
tected, and  their  plan  is  slightly  convex  on  the  up-stream  side. 


a 


Fig.  247— Represents  a  section  of  the  timber  wing-dams  on  the  Po,  formed  of  plank  nailed  on 

the  inclined  pieces  of  the  ribs. 
a  b  and  b-c,  inclined  faces  of  the  dam,  the  first  making  an  aJigle  of  63°,  and  the  second  of  23** 

with  the  horizon. 
d  and  e  pieces  of  the  rib. 
/  and  g  horizontal  pieces  connecting  the  ribs. 

Wing-dams  of  the  ordinary  form  and  construction  are  now 
regarded,  from  the  experience  of  a  long  series  of  years  on  the 
Ehine,  and  some  other  rivers  in  Europe,  as  little  seviceable, 
if  not  positively  hurtful,  as  a  river  improvement,  and  the 
abandonment  of  their  use  has  been  strongly  urged  by  engi- 
neers in  France. 

The  action  of  the  current  against  the  side  of  the  dam 
causes  whirls  and  counter-currents,  which  are  found  to  un- 
dermine the  base  of  the  dam,  and  the  bank  adjacent  to  it. 
Shallows  and  bars  are  formed  in  the  bed  of  the  stream,  near 
the  dam,  by  the  debris  borne  along  by  the  current  after  it 
passes  the  dam,  giving  very  frequently  a  more  tortuous  coui-se 
to  the  channel  than  it  had  naturally  assumed  in  the  elbow. 


RIVEES. 


499 


The  best  method  yet  found  of  arresting  the  progress  of  an 
elbow  is  to  protect  the  concave  bank  by  a  facing  of  dry 
stone,  formed  by  throwing  in  loose  blocks  of  stone  along  the 
foot  of  the  bank,  and  giving  them  the  slope  they  naturally 
assume  when  thus  thrown  in. 

Wing-walls  were  put  into  the  Hudson  River  many  years 
since  for  the  purpose  of  removing  the  bars  and  improving 
the  stream  for  navigable  purposes.  The  result  has  been  that 
they  produced  a  scour  in  the  narrowed  part  of  the  stream 
which  removed  the  sand  and  other  materials  of  the  bar  to 
points  lower  down  in  the  stream  where  it  was  again  depos- 
ited ;  thus  removing  the  previous  obstruction  only  to  produce 
a  worse  one  in  a  new  place. 

Gen.  Totten,  in  an  able  report  to  the  Government  on  the 
improvement  of  rivers  having  bars,  showed  very  clearly  the 
error  of  attempting  to  improve  rivers  by  means  of  wing-dams. 
He  recommended  the  establishment  of  a  uniform  channel  by 
longitudinal  dikes,  made  of  continuous  piles  or  of  walls  of 
masonry.  This  plan  has  been  adopted  more  recently  and 
with  good  results. 

835.  Elbows  upon  most  rivers  finally  reach  that  state  of 
development  in  which  the  wear  upon  the  concave  side,  from 
the  action  of  the  current,  will  be  entirely  suspended,  and  the 
regimen  of  the  river  at  these  points  will  remain  stable.  This 
state  will  depend  upon  the  nature  of  the  soil  of  the  banks 
and  bed,  and  the  character  of  the  freshets.  From  observa- 
tions made  upon  the  Rhine,  it  is  stated  that  elbows,  with  a 
radius  of  curvature  of  nearly  3,000  yards,  preserve  a  fixed 
regimen ;  and  that  the  banks  of  those  which  have  a  radius  of 
about  1,500  yards  are  seldom  injured,  if  properly  faced. 

836.  Attempts  have,  in  some  cases,  been  made  to  shorten 
and  straighten  the  course  of  a  river,  by  cutting  across  the 
tongue  of  land  that  forms  the  convex  bank  of  the  elbow,  and 
turning  the  water  into  a  new  channel.  It  has  generally  been 
found  that  the  stream  in  time  forms  for  itself  a  new  bed  of 
nearly  the  same  character  as  it  originally  had. 

837.  Bars.  To  obtain  a  sufticient  depth  of  water  over 
bars,  the  deposite  must  either  be  scooped  up  by  machinery, 
and  be  conveyed  away,  or  be  removed  by  giving  an  increased, 
velocity  to  the  current.  When  the  latter  plan  is  preferred, 
an  artificial  channel  is  formed,  by  contracting  the  natural 
way,  confining  it  between  two  low  dikes,  which  should  rise 
only  a  little  above  the  ordinary  level  of  low  water,  so  that  a 
sufBcient  outlet  may  be  left  for  the  water  during  the  season 
of  freshets,  by  allowing  it  to  flow  over  the  dams. 


500 


CIVIL  ENGINEERING. 


If  tliG  river  separates  into  several  channels  at  the  bar,  dams 
should  be  built  across  all  except  the  main  channel,  so  that  by 
throwing  the  whole  of  the  Avater  into  it  the  effects  of  the  cur- 
rent may  be  greater  upon  the  bed. 

Tlie  longitudinal  dikes,  between  which  the  main  channel 
is  confined,  should  be  placed  as  nearly  as  practicable  in  the 
direction  Avhich  the  channel  has  naturally  assumed.  If  it  be 
deemed  advisable  to  change  the  position  of  the  cliannel,  it 
should  be  shifted  to  that  side  of  the  bed  which  will  yield  most 
readily  to  the  action  of  the  current.  , 

838.  In  situations  where  large  reservoirs  can  be  formed 
near  the  bar,  tlie  water  from  them  may  be  used  for  removing 
it.  For  this  purpose  an  ontlet  is  made  from  the  reservoir,  in 
the  direction  of  the  bar,  which  is  closed  by  a  gate  that  turns 
upon  a  vertical  axis,  and  is  so  arranged  that  it  can  be  sudden- 
ly thrown  open  to  let  off  the  water.  The  chase  of  water 
formed  in  this  way  sweeping  over  the  bar  will  prevent  the 
accumulation  of  deposites  upon  it.  This  plan  is  frequently 
resorted  to  in  Europe  for  the  removal  of  deposites  that  accu- 
mulate at  the  mouth  of  harbors  in  those  localities  wliere,  from 
the  height  to  which  the  tide  rises,  a  great  head  of  water  can  be 
obtained  in  the  reservoirs. 

839.  In  the  improvement*  of  the  mouths  of  rivers  wdiich 
empty  into  the  sea  through  several  channels,  no  obstruction 
should  be  placed  to  the  free  ingress  of  the  tides  through  all 
the  channels.  If  the  main  channel  is  subject  to  obstructions 
from  deposites,  dams  should  be  built  across  the  secondary 
channels,  which  may  be  so  arranged  with  cuts  through  them, 
closed  by  gates,  that  the  flood-tide  will  meet  wdth  no  obstruc- 
tion from  the  gates,  while  the  ebb-tide,  causing  the  gates  to 
close,  will  be  forced  to  recede  through  the  main  channel, 
which,  in  this  way,  will  be  daily  scoured,  and  freed  from  de- 
posites by  the  ebb  current.  The  same  object  may  be  effected 
by  building  dams  without  inlets  across  the  secondary  channels, 
giving  them  such  a  height  that  at  a  certain  stage  of  the  flood- 
tide  the  water  will  flow  over  them  and  fill  the  channels  above 
the  dams.  The  portion  of  water  thus  dammed  in  will  be 
forced  through  the  main  channel  at  the  ebb. 

840.  When  the  bed  is  obs-tructed  by  rocks,  it  may  be  deep- 
ened by  blasting  the  rocks,  and  removing  the  fragments  with 
the  assistance  or  the  diving-bell  and  other  machinery. 

841.  In  some  of  our  rivers,  obstructions  of  a  very  danger- 
ous character  to  boats  are  met  with,  in  the  trunks  of  large 
trees  wdiich  are  embedded  in  the  bottom  at  one  end,  while  tlie 
other  is  near  the  surface ;  they  are  termed  snags  and  sawyers 


EIVERS. 


501 


by  the  boatmen.  These  obstructions  have  been  very  success- 
fully removed,  within  late  years,  by  means  of  machiner}^,  and 
by  propelling  two  heavy  boats,  moved  by  steam,  which  are 
connected  by  a  strong  beam  across  their  bows,  so  that  the 
beam  will  strike  the  snag,  and  either  break  it  off  near  the 
bottom  or  uproot  it.  Other  obstructions,  termed  rafts,  form- 
ed by  the  accumulation  of  drift-wood  at  points  of  a  river's 
course,  are  also  found  in  some  of  our  western  rivers.  These 
are  also  in  process  of  removal,  by  cutting  through  them  by 
various  means  which  have  been  found  successful. 

842.  Slack-water  Navigation.  When  the  general  depth 
of  water  in  a  river  is  insufficient  for  the  draught  of  boats  of 
the  most  suitable  size  for  the  trade  on  it,  an  improvement, 
termed  slack-water  or  lock  and  dam  navigation^  is  resorted 
to.  This  consists  in  dividing  the  course  into  several  suitable 
ponds,  by  forming  dams  to  keep  the  water  in  the  pond  at  a 
constant  head ;  and  by  passing  from  one  pond  to  another  by 
locks  at  the  ends  of  the  dams. 

843.  The  position  of  the  dams,  and  the  number  requisite, 
will  depend  upon  the  locality.  In  streams  subject  to  heavy 
freshets,  it  will  generally  be  advisable  to  place  the  dams  at 
the  widest  parts  of  the  bed,  to  obtain  the  greatest  outlet  for  the 
water  over  the  dam.  The  dams  may  be  built  either  in  a 
straight  line  between  the  banks,  and  perpendicular  to  the 
thread  of  the  current,  or  they  may  be  in  a  straight  line  ob- 
lique to  the  current,  or  their  plan  may  be  convex,  the  convex 
surface  being  up-stream,  or  it  may  be  a  broken  line  present- 
ing an  angle  up-stream.  The  three  last  forms  offer  a  greater 
outlet  than  the  first  to  the  water  that  flows  over  the  dam,  but 
are  more  liable  to  cause  injury  to  the  bed  below  the  stream, 
from  the  oblique  direction  which  the  current  may  receive, 
arising  from  the  form  of  the  dam  at  top. 

844.  The  cross  section  of  a  dam  is  usually  trapezoidal,  the 
face  up-stream  being  inclined,  and  the  one  down-stream 
either  vertical  or  inclined.  When  the  down-stream  face  is 
vertical,  the  velocity  of  the  water  which  flows  over  the  dam 
is  destroyed  by  the  shock  against  the  water  of  the  pond  below 
the  dam,  but  whirls  are  formed  which  are  more  destructive 
to  the  bed  than  would  be  the  action  of  the  current  upon  it 
along  the  inclined  face  of  a  dam.  In  all  cases  the  sides  and 
bed  of  the  stream,  for  some  distance  below  the  dam,  should 
be  protected  from  the  action  of  the  current  by  a  facing  of  dry 
stone,  timber,  or  any  other  construction  of  sufficient  dura- 
bility for  the  object  in  view. 

845.  The  dams  should  receive  a  sufficient  height  only  to 


502 


CIVIL  ENGINEERING. 


maintain  the  requisite  depth  of  water  in  the  ponds  for  the 
purj)oses  of  navigation.  Any  material  at  hand,  offering  suffi- 
cient durability  against  the  action  of  the  water,  may  be  re- 
sorted to  in  their  construction.  Dams  of  alternate  layers  of 
brush  and  gravel,  with  a  facing  of  plank,  fascines,  or  dry 
stone,  answer  very  well  in  gentle  currents.  If  the  dam  is  ex- 
posed to  heavy  freshets,  to  shocks  of  ice,  and  other  heavy 
floating  bodies,  as  drift-wood,  it  would  be  more  prudent  to 
form  it  of  dry  stone  entirely, or  of  crib-work  filled  with  stone; 
or,  if  the  last  material  cannot  be  obtained,  of  a  solid  crib-work 
alone.  If  the  dam  is  to  be  made  water-tight,  sand  and  gravel 
in  sufficient  quantity  may  be  thi'own  in  against  it  in  the 
upper  pond.  The  points  where  the  dam  joins  the  banks, 
which  are  termed  the  roots  of  the  dam,  require  particular  at- 
tention to  prevent  the  water  from  filtering  around  them. 
The  ordinary  precaution  for  this  is  to  build  the  dam  some 
distance  back  into  the  banks. 

846.  The  safest  means  of  communication  between  the 
ponds  is  by  an  ordinary  lock.  It  should  be  placed  at  one 
extremity  of  the  dam,  an  excavation  in  the  bank  being  made 
for  it,  to  secure  it  from  damage  by  floating  bodies  brought 
down  by  the  current.  The  sides  of  the  lock  and  a  portion  of 
the  dam  near  it  should  be  raised  sufficiently  high  to  prevent 
them  from  being  overflowed  by  the  heaviest  freshets.  When 
the  height  to  which  the  freshets  rise  is  great,  the  leaves  of 
the  head  gates  should  be  formed  of  two  parts,  as  a  single  leaf 
would,  from  its  size,  be  too  unwieldy,  the  lower  portion  being 
of  a  suitable  height  for  the  ordinary  manoeuvres  of  the  lock ; 
the  upper,  being  used  only  during  the  freshets,  are  so  ar- 
ranged that  their  bottom  cross  pieces  shall  rest,  when  the 
gates  are  closed,  against  the  top  of  the  lower  portion  s.  An 
arrangement  somewhat  similar  to  this  may  be  made  for  the 
tail  gates,  when  the  lifts  of  the  locks  are  great,  to  avoid  the 
difficulty  of  manoeuvring  very  high  gates,  by  permanently 
closing  the  upper  part  of  the  entrance  to  the  lock  at  the  tail 
gates,  either  by  a  wall  built  between  the  side  walls,  or  by  a 
permanent  framework,  below  which  a  sufficient  height  is  left 
for  the  boats  to  pass. 

847.  A  common,  but  unsafe  method  of  passing  from  one 
pond  to  another,  is  that  which  is  t^rrnQdi  flashing  y'  it  consists 
of  a  sluice  in  the  dam,  which  is  opened  and  closed  by  means 
of  a  gate  revolving  on  a  A^ertical  axis,  which  is  so  arranged 
that  it  can  be  manoeuvred  with  ease.  One  plan  for  this  pur- 
pose is  to  divide  the  gate  into  two  unequal  parts  by  an  axis, 
and  to  place  a  valve  of  such  dimensions  in  the  greater,  that 


EIVERS. 


503 


when  opened  the  surface  against  which  the  water  presses 
shall  be  less  than  that  of  the  smaller  part.  The  play  of  the 
gate  is  thns  rendered  very  simple ;  when  the  valve  is  shut, 
the  pressure  of  water  on  the  larger  surface  closes  it  against 
the  sides  of  the  sluice;  when  the  valve  is  opened,  the  gate 
swings  round  and  takes  a  position  in  the  direction  of  the  cur- 
rent. Yarious  other  plans  for  flashing,  on  similar  principles, 
are  to  be  met  with. 

848.  AVhen  the  obstruction  in  a  river  cannot  be  overcome 
by  any  of  the  preceding  means,  as  for  example  in  those  con- 
siderable descents  in  the  bed  known  as  rapids,  where  the 
water  acquires  a  velocity  so  great  that  a  boat  can  neither 
ascend  nor  descend  with  safety,  resort  must  be  had  to  a  canal 
for  the  purpose  of  uniting  its  navigable  parts  above  and 
below  the  obstruction. 

The  general  direction  of  the  canal  will  be  parallel  to  the 
bed  of  the  river.  In  some  cases  it  may  occupy  a  part  of  the 
bed  by  forming  a  dike  in  the  bed  parallel  to  the  bank,  and 
sufficiently  far  from  it  to  give  the  requisite  width  to  the  canal. 
Whatever  position  the  canal  may  occupy,  every  precaution 
should  be  taken  to  secure  it  from  damage  by  freshets. 

849.  A  lock  will  usually  be  necessary  at  each  extremity  of 
the  canal  where  it  joins  the  river.  The  positions  for  the  ex- 
treme locks  should  be  carefully  chosen,  so  that  the  boats  can 
at  all  times  enter  them  with  ease  and  safety.  The  locks 
should  be  secured  by  guard  gates  and  other  suitable  means 
from  freshets ;  and  if  they  are  liable  to  be  obstructed  by  de- 
posites,  arrangements  should  be  made  for  their  removal  either 
by  a  chase  of  water,  or  by  machinery. 

If  the  river  should  not  present  a  sufficient  depth  of  water 
at  all  seasons  for  entering  the  canal  from  it,  a  dam  will  be 
required  at  some  point  near  the  lock  to  obtain  the  depth  re- 
quisite. 

It  may  be  advisable  in  some  cases,  instead  of  placing  the 
extreme  locks  at  the  outlets  of  the  canal  to  the  river,  to  form 
a  capacious  basin  at  each  extremity  of  the  canal  between  the 
lock  and  river,  where  the  boats  can  lie  in  safety.  The  outlets 
from  the  basins  to  the  rivers  may  either  be  left  open  at  all 
times,  or  else  guard  gates  may  be  placed  at  them  to  shut  off 
the  water  during  freshets. 


504 


CIVIL  ENGLNEEEING. 


CIIAPTEE  X. 
SEACOAST  IMPROVEMENTS. 

850.  The  following  subdivisions  may  be  made  of  the  works 
belonging  to  this  class  of  improvements :  1st.  Artificial 
Koadsteads.  2d.  The  works  required  for  natural  and  ar- 
tificial Harbors.  3d.  The  works  for  the  j)i"otection  of  the 
seacoast  against  the  action  of  the  sea. 

851.  Before  adopting  any  definitive  plan  for  the  improve- 
ment of  the  seacoast  at  any  point,  the  action  of  the  tides, 
currents,  and  waves  at  that  point  must  be  ascertained. 

852.  The  theory  of  tides  is  well  understood  ;  their  rise  and 
duration,  caused  by  the  attraction  of  the  sun  and  moon,  are 
also  dependent  on  the  strength  and  direction  of  the  wind, 
and  the  conformation  of  the  shore.  Along  our  own  sea- 
board, the  highest  tides  vary  greatly  between  the  most 
southern  and  northern  parts.  At  Eastport,  Me.,  the  highest 
tides,  when  not  affected  by  the  wind,  vary  between  twenty- 
five  and  thirty  feet  above  the  ordinary  low  water.  At  Bos- 
ton they  rise  from  eleven  to  twelve  feet  above  the  same 
point,  under  similar  circumstances ;  and  from  New  York, 
lollowing  the  line  of  the  seaboard  to  Florida,  they  seldom 
rise  above  five  feet. 

853.  Currents  are  principally  caused  by  the  tides,  assisted, 
in  some  cases,  by  the  wind.  The  theory  of  their  action  is 
simple.  From  the  main  current,  which  sweeps  along  the 
coast,  secondary  currents  proceed  into  the  hays^  or  indenta- 
tions, in  a  line  more  or  less  direct,  until  they  strike  some 
point  of  the  sliore,  from  which  they  are  deflected,  and  fre- 
quenytly  se2;)arate  into  several  others,  the  main  branch  follow- 
ing tlie  general  direction  which  it  had  when  it  struck  the 
shore,  and  the  others  not  unfrequently  taking  an  opposite 
direction,  forming  what  are  termed  counter  currents,  and,  at 
points  where  the  opposite  currents  meet,  that  rotary  motion 
of  the  w^ater  known  as  whirlpools.  The  action  of  currents 
on  the  coast  is  to  wear  it  away  at  those  points  against  which 
they  directly  impinge,  and  to  transport  the  debris  to  other, 
points,  thus  forming,  and  sometimes  removing,  natural  ob- 
structions to  navigation.  These  continual  changes,  caused 
by  currents,  make  it  extremely  difticult  to  foresee  their  effects. 


SEA  COAST  BIPKOVEMENTS. 


605 


and  to  foretell  the  consequences  which  will  arise  from  any 
cliange  in  the  direction,  or  the  intensity  of  a  current,  occa- 
sioned by  artificial  obstacles. 

854.  A  good  theory  of  waves,  which  shall  satisfactorily 
explain  all  their  phenomena,  is  still  a  desideratum  in  science. 
It  is  known  that  they  are  produced  b}''  winds  acting  on  the 
surface  of  the  sea ;  but  how  far  this  action  extends  below 
the  surface  and  what  are  its  effects  at  various  depths,  are 
questions  that  remain  to  be  answered.  The  most  commonly 
received  theory  is,  that  a  wave  is  a  simple  oscillation  of  the 
water,  in  which  each  particle  rises  and  falls,  in  a  vertical 
line,  a  certain  distance  during  each  oscillation,  without  re- 
ceiving any  motion  of  translation  in  a  horizontal  direction. 
It  has  been  objected  to  this  theory  that  it  fails  to  explain 
many  phenomena  observed  in  connection  with  waves. 

In  a  recent  French  work  on  this  subject,  its  author.  Colonel 
Emy,  an  engineer  of  high  standing,  combats  the  received 
theory;  and  contends  that  the  particles  of  water  receive  also 
a  motion  of  translation  horizontally,  which,  with  that  of  as- 
cension, causes  the  particles  to  assume  an  orbicular  motion, 
each  particle  describing  an  orbit,  which  he  supiposes  to  be 
elliptical.  He  farther  contends,  that  in  this  manner  the  par- 
ticles at  the  surface  communicate  their  motion  to  those  just 
below  them,  and  these  again  to  the  next,  and  so  on  down- 
ward, the  intensity  decreasing  from  the  surface,  without, 
however,  becoming  insensible  at  even  very  considerable 
depths ;  and  that,  in  this  w^ay,  owing  to  the  reaction  from 
the  bottom,  an  immense  volume  of  water  is  propelled  along 
the  bottom  itself,  with  a  motion  of  translation  so  powerful  as 
to  overthrow  obstacles  of  the  greatest  strength  if  directly 
opposed  to  it.  From  this  he  argues  that  walls  built  to  resist 
the  shock  of  the  waves  should  receive  a  very  great  batir  at 
the  base,  and  that  this  batir  should  be  gradually  decreased 
upward,  until,  towards  the  to]3,  the  wall  should  project  over, 
thus  presenting  a  concave  surface  at  top  to  throw  the  water 
back.  By  adopting  this  form,  he  contends  that  the  mass  of 
water,  which  is  rolled  forward,  as  it  were,  on  the  bottom, 
when  it  strikes  the  face  of  the  wall,  will  ascend  along  it,  and 
thus  gradually  lose  its  momentum.  These  views  of  Colonel 
Emy  have  been  attacked  by  other  engineers,  who  have  had 
opportunities  to  observe  the  same  phenomena,  on  the  ground  { 
that  they  are  not  supported  by  facts ;  and  the  question  still 
remains  undecided.  It  is  certain,  from  experiments  made 
by  the  author  quoted  upon  walls  of  the  form  here  described, 
that  they  seem  to  answer  fully  their  intended  purpose. 


506 


CIVIL  ENGINEERING. 


855.  Roadsteads.  The  term  roadstead  is  applied  to  an 
indentation  of  the  coast,  where  vessels  may  ride  securely  at 
anchor  nnder  all  circnmstances  of  weather.  If  the  indenta- 
tion is  covered  by  natural  projections  of  the  land,  or  capes^ 
from  the  action  of  the  winds  and  waves,  it  is  said  to  be  land- 
locked j  in  the  contrary  case,  it  is  termed  an  open  roadstead. 

The  anchorage  of  open  roadsteads  is  often  msecure,  owing 
to  violent  winds  setting  into  them  from  the  sea,  and  occasion- 
ing high  waves,  which  are  very  straining  to  the  moorings.' 
The  remedy  applied  in  this  case  is  to  place  an  obstruction 
near  the  entrance  of  the  roadstead,  to  break  the  force  of  the 
waves  from  the  sea.  These  obstructions,  termed  hreaJcwaters, 
are  artificial  islands  of  greater  or  less  extent,  and  of  variable 
form,  according  to  the  nature  of  the  case,  made  by  throwing 
heavy  blocks  oi  stone  into  the  sea,  and  allowing  them  to  take 
their  own  bed. 

The  first  great  work  of  this  kind  undertaken  in  modern 
times,  was  the  one  at  Cherbourg  in  France,  to  cover  the  road- 
stead in  front  of  tliat  town.  After  some  trials  to  break  the 
effects  of  the  waves  on  the  roadstead  by  placing  large  conical- 
shaped  structures  of  timber  filled  with  stones  across  it,  which 
resulted  in  failure,  as  these  vessels  were  completely  destroyed 
by  subsequent  storms,  the  plan  was  adopted  of  forming  a 
breakwater  by  throwing  in  loose  blocks  of  stone,  and  allow- 
ing the  mass  to  assume  the  form  produced  by  the  action  of 
the  waves  upon  its  surface.  The  subsequent  experience  of 
many  years,  during  which  this  work  has  been  exposed  to  the 
most  violent  tempests,  has  shown  that  the  action  of  the  sea 
on  the  exposed  surface  is  not  very  sensible  at  this  locality  at 
a  depth  of  about  20  feet  below  the  water  level  of  the  lowest 
tides,  as  the  blocks  of  stone  forming  this  part  of  tlie  break- 
w^ater,  some  of  which  do  not  average  over  40  lbs.  in  weight, 
have  not  been  displaced  from  the  slope  the  mass  first  as- 
sumed, which  was  somewhat  less  than  one  perpendicular  to 
one  base.  From  this  point  upwards,  and  particularly  be- 
tween the  levels  of  high  and  low  water,  the  action  of  the 
waves  has  been  very  powerful  at  times,  during  violent  gales, 
displacing  blocks  of  several  tons  weight,  throwing  them  over 
the  top  of  the  breakwater  upon  the  slope  towards  the  shore. 
Wherever  this  part  of  the  surface  has  been  exposed  the 
blocks  of  stone  have  been  gradually  worn  down  by  the  action 
of  the  waves,  and  the  slope  has  become  less  and  less  steep, 
from  year  to  year,  until  finally  the  surface  assumed  a  slightly 
concave  slope,  which,  at  some  points,  was  as  great  as  ten 
base  to  one  perpendicular. 


SEACOAST  IMPROVEMENTS. 


607 


The  experience  acquired  at  this  work  has  conclusively 
shown  that  breakwaters,  formed  of  the  heaviest  blocks  of 
loose  stone,  ai-e  always  liable  to  damage  in  heavy  gales  when 
the  sea  breaks  over  them,  and  that  the  only  means  of  secur- 
ing them  is  by  covering  the  exposed  surface  with  a  facing  of 
heavy  blocks  of  hammered  stone  carefully  set  in  hydraulic 
cement. 

As  the  Cherbourg  breakwater  is  intended  also  as  a  military 
construction,  for  the  protection  of  the  roadstead  against  an 
enemy's  fleet,  the  cross  section  shown  (in  Fig.  248)  has  been 
adopted  for  it.  Profiting  by  the  experience  of  many  years' 
observation,  it  was  decided  to  construct  the  work  that  forms 
the  cannon  battery  of  solid  masonry  laid  on  a  thick  and  broad 
bed  of  beton.  The  top  surface  of  the  breakwater  is  covered 
with  heavy  loose  blocks  of  stone,  and  the  foot  of  the  wall  on 
the  face  is  protected  by  large  blocks  of  artificial  stone  formed 
of  beton.  The  top  of  the  battery  is  about  12  feet  above  the 
highest  water  level. 


Fig.  248— Represents  a  section  of  the  Cherbourg  breakwater. 

A,  mass  of  stone. 

B,  battery  of  masonry. 


The  next  work  of  the  kind  was  built  to  cover  the  roadstead 
of  Plymouth  in  England.  Its  cross  section  was,  at  first,  made 
with  an  interior  slope  of  one  and  a  half  base  to  one  perpen- 
dicular, and  an  exterior  slope  of  only  three  base  to  one  per- 
pendicular ;  but  from  the  damage  it  sustained  in  the  severe 
tempests  in  the  winter  of  1816-17,  it  is  thought  that  its  ex- 
terior slope  was  too  abrupt. 

A  work  of  the  same  kind  is  still  in  process  of  construction 
on  our  coast,  off  the  mouth  of  the  Delaware.  The  same  cross 
section  has  been  adopted  for  it  as  in  the  one  at  Cherbourg. 

All  of  tliese  works  were  made  in  the  same  way,  discharg- 
ing the  stone  on  the  spot,  from  vessels,  and  allowing  it  to 
take  its  own  bed,  except  for  the  facing,  where,  when  practi- 
cable, the  blocks  were  carefully  laid,  so  as  to  present  a  uni- 
form surface  to  the  waves.    The  interior  of  the  mass,  in  each 


508 


CIVIL  EJSIGINEEEING. 


case,  lias  been  formed  of  stone  in  small  blocks,  and  the  facing 
of  very  large  blocks.  It  is  thought,  however,  that  it  would 
be  more  prudent  to  form  the  whole  of  large  blocks,  because, 
were  the  exterior  to  suffer  damage,  and  experienccshows  that 
the  heaviest  blocks  yet  used  have  at  times  been  displaced  l)y 
the  shock  of  the  waves,  the  interior  would  still  present  a  great 
obstacle. 

From  the  foregoing  details,  respecting  the  cross  sections  of 
breakwaters,  which  from  experiment  have  been  found  to 
answer,  the  proper  form  and  dimensions  of  the  cross  section 
in  similar  cases  may  be  arranged.  As  to  the  j^lan  of  such 
works,  it  must  depend  on  the  locality.  The  position  of  the 
breakwater  should  be  chosen  with  regard  to  tlie  direction  of 
the  heaviest  swells  from  the  sea  into  the  roadstead, — the 
action  of  the  current,  and  that  of  waves.  The  part  of  the 
roadstead  which  it  covers  should  afford  a  proper  depth  of 
water,  and  secure  anchorage  for  vessels  of  the  largest  class, 
during  the  most  severe  storms  ;  and  vessels  should  be  able  to 
double  the  breakwater  under  all  circumstances  of  wind  and 
tide.  Such  a  position  should,  moreover,  be  chosen  tliat  there 
will  be  no  liability  to  obstructions  being  formed  within  the 
roadstead,  or  at  any  of  its  outlets,  from  the  change  in  the 
current  which  may  be  made  by  the  breakwater. 

856.  The  difficulty  of  obtaining  very  heavy  blocks  of  stone, 
as  well  as  their  great  cost,  has  led  to  the  suggestion  of  substi- 
tuting for  them  blocks  of  artificial  stone,  formed  of  concrete, 
which  can  be  made  of  ai^y  shape  and  size  desirable.  This 
plan  has  been  tried  with  success  in  several  instances,  particu- 
larly in  a  jetty  or  mole,  at  Algiers,  constructed  by  the  French 
government.  The  beton  for  a  portion  of  this  work  was  placed 
in  large  boxes,  the  sides  of  which  were  of  wood,  shaped  at 
bottom  to  correspond  to  the  irregularities  of  the  bottom  on 
which  the  beton  was  to  be  spread.  The  bottom  of  the  box 
was  made  of  strong  canvas  tarred'.  These  boxes  were  first 
sunk  in  tlie  position  for  which  they  were  constructed,  and  then 
filled  with  the  beton. 

857.  Harbors.  The  term  harbor  is  applied  to  a  secure  an- 
chorage of  a  more  limited  capacity  than  a  roadstead,  and 
therefore  offering  a  safer  refuge  dnring  boisterous  weather. 
Harbors  are  either  nataral  or  artificial. 

858.  An  artificial  harbor  is  usually  formed  by  enclosing  a 
space  on  the  coast  between  two  arms,  or  dikes  of  stone,  or  of 
wood,  termed  jetties^  which  project  into  the  sea  from  the 
shore,  in  such  a  way  as  to  cover  the  harbor  from  the  action  of 
the  wind  and  waves. 


SEACOAST  II»rPEOYEMENTS. 


509 


859.  The  plan  of  each  jetty  is  curved,  and  the  space  enclosed 
by  the  two  will  depend  on  the  nnmber  of  vessels  which  it  may 
be  supposed  will  be  in  the  harbor  at  the  same  time.  The  dis- 
tance between  the  ends,  or  heads,  of  the  jetties  which  forms 
the  month  of  the  harbor,  will  also  depend  on  local  circnm- 
stances  ;  it  should  seldom  be  less  than  one  hundred  yards,  and 
generally  need  not  be  more  than  five  hundred.  There  are 
certain  winds  at  every -point  of  a  coast  which  are  more  un- 
favorable than  others  to  vessels  entering  and  qutting  the  har- 
bor, and  to  the  tranquillity  of  its  water.  Oue  of  the  jetties 
should,  on  this  account,  be  longer  than  the  other,  and  be  so 
placed  that  it  will  both  break  the  force  of  the  heaviest  swells 
from  the  sea  into  the  month  of  the  harbor,  and  facilitate  the 
ingress  and  egress  of  vessels,  by  preventing  them  from  being 
driven  by  the  winds  on  the  other  jetty,  just  as  they  are  enter- 
ing or  quitting  the  mouth. 

860.  The  cross  section  and  construction  of  a  stone  jetty 
differ  in  nothing  from  those  of  a  breakwater,  except  that  the 
jetty  is  usually  wider  on  top,  thirty  feet  being  allowed,  as  it 
serves  for  a  wdiarf  in  nnloading  vessels.  The  head  of  the 
jetty  is  usually  made  circular,  and  considerably  broader  than 
the  other  parts,  as  it,  in  some  instances,  receives  a  lighthouse, 
and  a  battery  of  cannon.  It  should  be  made  with  great  care, 
of  large  blocks  of  stone,  well  united  by  iron  or  copper  cramps, 
and  the  exterior  courses  should  moreover  be  j)rotected  by 
fender  beams  of  heavy  timber  to  receive  the  shock  of  floating 
bodies. 

861.  Wooden  jetties  are  formed  of  an  open  framework  of 
heavy  timber,  the  sides  of  w^hich  are  covered  on  the  interior 
by  a  strong  sheeting  of  thick  plank.  Each  rib  of  the  frame 
(Fig.  249)  consists  of  two  inclined  pieces,  wdiich  form  the 
sides — of  an  upright  centre  piece, — and  of  horizontal  clamp- 
ing pieces,  which  are  notched  and  bolted  in  pairs  on  the 
inclined  and  upright  pieces ;  the  inclined  pieces  are  farther 
strengthened  by  struts,  wdiich  abut  against  them  and  the  up- 
right. The  ribs  are  connected  by  large  string-pieces,  laid 
horizontally,  wdiich  are  notched  and  bolted  on  the  inclined 
pieces,  the  nprights,  and  the  clamping  pieces,  at  their  points 
of  junction.  The  foundation,  on  which  this  framework  rests, 
consists  nsually  of  three  rows  of  large  piles  driven  nnder 
the  foot  of  the  inclined  pieces  and  the  uprights.  The  rows 
of  piles  are  firmly  connected  by  cross  and  longitudinal  beams 
notched  and  bolted  on  them ;  and  they  are,  moreover,  firmly 
united  to  the  framework  in  a  similar  manner.  The  interior 
sheeting  does  not,  in  all  cases,  extend  the  entire  length  of 


610 


CIVIL  enghteeeinq. 


the  sides,  but  open  spaces,  termed  clear-waySy  are  often  left, 
to  ^ive  a  free  passage  and  spread  to  the  waves  confined  be- 
tween the  jetties,  for  the  purpose  of  forming  smooth  water 
in  the  channel.  If  the  jetties  are  covered  at  their  back  with 
earth,  the  clear-ways  receive  the  form  of  inclined  planes. 

The  foundation  of  the  jetties  requires  particular  care, 
especially  when  the  channel  between  them  is  very  narrow. 
Loose  stone  thrown  around  the  piles  is  the  ordinary  construc- 
tion used  for  this  purpose ;  and,  if  it  be  deemed  necessary, 
the  bottom  of  the  entire  channel  may  be  protected  by  an 
apron  of  brush  and  loose  stone. 

The  top  of  the  jetties  is  covered  w^ith  a  flooring  of  thick 
plank,  w^hich  serves  as  a  wharf.  A  strong  hand-railing 
sliould  be  placed  on  each  side  of  the  flooring  as  a  protection 
against  accidents.  The  sides  of  jetties  have  been  variously 
inclined;  the  rtiore  usual  inclination  varies  between  three 
and  four  perpendicular  to  one  base. 

862.  Jetties  are  sometimes  built  out  to  form  a  passage  to 
a  natural  harbor,  which  is  either  very  much  exposed,  or 
subject  to  bars  at  its  mouth.  By  narrowing  the  passage  to 
the  harbor  between  the  jetties,  great  velocity  is  given  to  the 
current  caused  by  the  tide,  and  this  alone  will  free  the 
greater  part  of  the  channel  from  deposites.  But  at  the  head 
of  the  jetties  a  bar  will,  in  almost  every  case,  be  found  to 
accumulate,  from  the  current  alongshore,  which  is  broken 


SEACOAST  IMPEOVEMENTS. 


511 


by  the  jetties,  and  from  the  diminished  yelocity  of  the  ebbing 
tides  at  this  point.  To  remove  these  bars  resort  may  be  had, 
in  localities  where  they  are  left  nearly  dry  at  low  water,  to 
reservoirs,  and  sluices,  arranged  with  turning  gates,  like  those 
adverted  to  for  river  improvements.  The  reservoirs  are 
formed  by  excavating  a  large  basin  inshore,  at  some  suitable 
point  from  which  the  collected  water  can  be  directed,  with 
its  full  force,  on  the  bar.  The  basin  will  be  filled  at  fiood- 
tide,  and  when  the  ebb  commences  the  sluice  gates  will  be 
kept  closed  until  dead  low  water,  when  they  should  all  be 
opened  at  once  to  give  a  strong  water  chase. 

863.  In  harbors  where  vessels  cannot  be  safely  and  conve- 
niently moored  alongside  of  the  quays,  large  basins,  termed 
wet-docks^  are  formed,  in  which  the  water  can  be  kept  at  a 
constant  level.  A  wet-dock  may  be  made  either  by  an  in- 
shore excavation,  or  by  enclosing  a  part  of  the  harbor  with 
strong  water-tight  walls ;  the  first  is  the  more  usual  plan. 
The  entrance  to  the  basin  may  be  by  a  simple  sluice,  closed 
by  ordinary  lock  gates,  or  by  means  of  an  ordinary  lock. 
With  the  first  method  vessels  can  enter  the  basin  only  at 
high  tide ;  by  the  last  they  may  be  entered  or  passed  out  at 
any  period  of  the  tide.  The  outlet  of  the  lock  should  be 
provided  with  a  pair  of  guard  gates,  to  be  shut  against  very 
high  tides,  or  in  cases  of  danger  from  storms. 

864.  The  construction  of  the  locks  for  basins  differs  in 
nothing,  in  principle,  from  that  pursued  in  canal  locks.  The 
greatest  care  will  necessarily  be  taken  to  form  a  strong  mass 
free  from  all  danger  of  accidents.  The  gates  of  a  basin-lock 
are  made  convex  towards  the  head  of  water,  to  give  them 
more  strength  to  resist  the  great  pressure  upon  them.  They 
are  hung  and  manoeuvred  differently  from  ordinary  lock 
gates ;  the  quoin-post  is  attached  to  the  side  walls  in  the  usual 
way :  but  at  the  foot  of  the  mitre-post  an  iron  or  brass  roller 
is  attached,  which  runs  on  an  iron  roller  way,  and  thus  sup- 
ports that  end  of  the  leaf,  relieving  the  collar  of  the  quoin- 
post  from  the  strain  that  would  be  otherwise  thrown  on  it, 
besides  giving  the  leaf  an  easy  play.  Chains  are  attached  to 
each  mitre-post  near  the  centre  of  pressure  of  the  water,  and 
the  gate  is  opened,  or  closed,  by  means  of  windlasses  to  which 
the  other  ends  of  the  chains  are  fastened. 

865.  The  quays  of  wet-docks  are  usually  built  of  masonry. 
Both  brick  and  stone  have  been  used ;  the  facing  at  least 
should  be  of  dressed  stone.  Large  fender-beams  may  be  at- 
tached to  the  face  of  the  wall,  to  prevent  it  from  being 
brought  in  contact  with  the  sides  of  the  vessels.    The  cross 


512 


CrV^L  ENGINEERING. 


section  of  qnaj-walls  should  be  fixed  on  the  same  principles 
as  that  of  otlier  sustaining  walls.  It  might  be  prudent  to  add 
buttresses  to  the  back  of  the  wall  to  strengthen  it  against  the 
shocks  of  the  vessels. 

866.  Quay-walls  with  us  are  ordinarily  made  either  by 
forming  a  facing  of  heavy  round  or  square  piles  driven  in 
juxtaposition,  which  are  connected  by  horizontal  pieces,  and 
secured  froiii  the  pressure  of  the  earth  filled  in  behind  them 
by  land-ties  ;  or,  by  placing  the  pieces  horizontally  upon  each 
other,  and  securing  them  by  iron  bolts.  Land-ties  are  used 
to  counteract  the  pressure  of  the  earth  or  rubbish  which  is 
thrown  in  behind  them  to  form  the  surface  of  the  quay. 
Another  mode  of  construction,  which  is  found  to  be  strong 
and  durable,  is  in  use  in  our  Eastern  seaports.  It  consists  in 
making  a  kind  of  crib- work  of  large  blocks  of  granite,  and 
filling  in  with  earth  and  stone  rubbish.  The  bottom  course 
of  the  crib  may  be  laid  on  the  bed  of  the  river,  if  it  is  firm 
and  horizontal ;  in  the  contrary  case  a  strong  grillage,  termed 
a  cradle^  must  be  made,  and  be  sunk  to  receive  the  stone  work. 
The  top  of  the  cradle  should  be  horizontal,  and  the  bottom 
should  receive  the  same  slope  as  that  of  the  bed,  in  order  that 
when  the  stones  are  laid  they  ma}^  settle  horizontally. 

867.  Dikes.  To  protect  the  lowlands  bordering  the  ocean 
from  inundations,  dikes,  constructed  of  ordinary  earth,  and 
faced  towards  the  sea  with  some  material  which  will  resist 
the  action  of  the  current,  are  usually  resorted'to. 

The  Dutch  dikes,  by  means  of  which  a  large  extent  of 
country  has  been  reclaimed  and  protected  from  the  sea,  are 
the  most  remarkable  structures  of  this  kind  in  existence.  The 
cross  section  of  those  dikes  is  of  a  trapezoidal  form,  the  width 
at  top  averaging  from  four  to  six  feet,  the  interior  slope  being 
the  same  as  the  natural  slope  of  the  earth,  and  the  exterior 
slope  varying,  according  to  circumstances,  between  three  and 
twelve  base  to  one  per2:)endicular.  The  top  of  the  dike,  for 
perfect  safety,  should  be  about  six  feet  above  the  level  of  the 
highest  spring  tides,  although,  in  many  places,  they  are  only 
two  or  three  above  this  level. 

The  earth  for  these  dikes  is  taken  from  a  ditch  inshore,  be- 
tween which  and  the  foot  of  the  dike  a  space  of  about  twenty 
feet  is  left  which  answers  for  a  road.  The  exterior  slope  is  va- 
riously faced,  according  to  the  means  at  hand,  and  the  charac- 
ter of  the  current  and  waves  at  the  point.  In  some  cases,  a 
strong  straw  thatch  is  put  on,  and  firmly  secured  by  pickets, 
or  other  means  ;  in  others,  a  layer  of  fascines  is  spread  over  the 
thatch,  and  is  strongly  picketed  to  it  the  ends  of  the  pickets 


SE.\  COAST  IMPEOVEMENTS. 


513 


being  allowed  to  project  out  about  eighteen  inches,  so  that 
they  can  receive  a  wicker-work  formed  by  interlacing  them^ 
with  twigs,  the  spaces  between  this  wicker-work  being  filled 
with  broken  stone ;  this  forms  a  very  durable  and  strong  facing, 
which  resists  not  only  the  action  of  the  current,  but,  by  its 
elasticity,  the  shocks  of  the  heaviest  waves. 

The  foot  of  the  exterior  slope  requires  peculiar  care  for  its 
protection ;  the  shore,  for  this  jDurpose,  is  in  some  places  cov- 
ered with  a  thick  apron  of  brush  and  gravel  in  alternate  layers, 
to  a  distance  of  one  hundred  yards  into  the  water  from  the  foot 
of  the  slope. 

On  some  parts  of  the  coast  of  France,  where  it  has  been 
found  necessary  to  protect  it  from  encroachments  of  the  sea, 
a  cross  section  has  been  given  to  the  dikes  towards  the  sea, 
of  the  same  form  as  the  one  which  the  shore  naturally  takes 
from  the  action  of  the  waves.  The  dikes  in  other  respects 
are  constructed  and  faced  after  the  manner  which  has  been 
so  long  in  practice  in  Holland. 

868.  Groins.  Constructions,  termed  groins,  are  used  when- 
ever it  becomes  necessary  to  check  the  effect  of  the  current 
along  the  shore,  and  cause  deposites  to  be  formed.  These  are 
artificial  ridges  which  rise  a  few  feet  only  above  the  surface 
of  the  beach,  and  are  built  out  in  a  direction  either  perpen- 
dicular to  that  of  the  shore,  or  oblique  to  it.  They  are  con- 
structed either  of  clay,  which  is  well  rammed  and  protected 
on  the  surface  by  a  facing  of  fascines  or  stones ;  or  of  layers 
of  fascines ;  or  of  one  or  two  rows  of  short  piles  driven  in 
juxtaposition;  or  any  other  means  that  the  locality  may  fur- 
nish may  be  resorted  to ;  the  object  being  to  interpose  an 
obstacle,  which,  breaking  the  force  of  the  current,  will  occa- 
sion a  deposite  near  it,  and  thus  gradually  cause  the  shore  to 
gain  upon  the  sea. 

869.  Sea-walls.  When  the  sea  encroaches  upon  the  land, 
forming  a  steep  bluff,  the  face  of  which  is  gradually  woi-n 
away,  a  wall  of  masonry  is  the  only  means  that  will  afford  a 
permanent  protection  against  this  action  of  the  waves.  Walls 
made  fortius  object  are  iQvmQdi  sea-walls.  The  face  of  a  sea- 
wall should  be  constructed  of  the  most  durable  stone  in  large 
blocks.  The  backing  may  be  of  rubble  or  of  beton.  The 
whole  work  should  be  laid  with  hydraulic  mortar. 

22* 


END. 


\ 


New  York,  August,  1873. 


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DOWNING  &       HINTS  TO  PERSONS  ABOUT  BUILDING  IN  THE 
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adapted  to  North  America.  By  A.  J.  Do^v^ling.  Containing 
a  revised  List  of  Trees,  Shrubs,  Plants,  and  the  most  recent 
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"This  work  commends  itself  by  its  practical  excellence." 

"It  is  a  valuable  addition  to  the  library  of  the  architect,  and  almost  indiiipensable 
to  every  scientific  |naster-mechanic." — E.  R.  Journal. 

HOLLY  CARPENTERS'  AND  JOINERS'  HAND-BOOK,  contain- 
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(I 

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LECTURES  ON  ARCHITECTURE  AND  PAINTING. 
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A  TREATISE  ON  THE  RESISTANCE  OF  MA- 
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A  TREATISE  ON  ASTRONOMY,  SPHERICAL  AND 
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BIBLES,  &c. 

BACSTER.  THE  COMMENTARY  WHOLLY  BIBLICAL.  Contents: 

— The  Commentary  :  an  Exposition  of  the  Old  and  New  Tes- 
taments in  the  very  words  of  Scripture.  2264  pp.  II.  An 
outline  of  the  Geography  and  History  of  the  Nations  men- 
tioned in  Scripture.  III.  Tables  of  Measures,  Weights,  and 
Coins.  IV.  An  Itinerary  of  the  Children  of  Israel  from 
Egypt  to  the  Promised  Land.  V.  A  Chronological  compara- 
tive Table  of  the  Kings  and  Prophets  of  Israel  and  Judah. 
VI.  A  Chart  of  the  World's  History  from  Adam  to  the  Third 
Century,  A.  D.  VII.  A  complete  Series  of  Illustrative  Maps. 
IX.  A  Chronological  Arrangement  of  the  Old  and  New  Tes- 
taments. X.  An  Index  to  Doctrines  and  Subjects,  with 
numerous  Selected  Passages,  quoted  in  full.  XI.  An  Index 
to  the  Names  of  Persons  mentioned  in  Scripture.  XII.  An 
Index  to  the  Names  of  Places  found  in  Scripture.  XIII. 
The  Names,  Titles,  and  Characters  of  Jesus  Christ  our  Lord, 
as  revealed  in  the  Scriptures,  methodically  arranged. 


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BLANK-PAGED    THE  HOLY  SCRIPTURES  OF  THE  OLD  AND  NEW 
BIBLE.        TESTAMENTS;  with  copious  references  to  parallel  and 
illustrative  passages,  and  the  alternate  pages  ruled  for  MS. 
notes. 

This  edition  of  the  Scriptures  contains  the  Authorized  Version,  illustrated  by  the 
references  of  "Bagster's  Polyglot  Bible,''  and  enriched  with  accurate  maps, 
useful  tables,  and  an  Index  of  Subjects. 

1  vol.  8vo,  half  morocco  $8  50 

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THE  TREASURY  Containing  the  authorized  English  version  of  the  Holy  Scriptures, 
BIBLE.  interleaved  with  a  Treasury  of  more  than  500,000  Parallel 

Passages  from  Canne,  Bro\vn,  Blayney,  Scott,  and  others. 
With  numerous  illustrative  notes. 

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JONES. 


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CHEMISTRY. 

CRAFTS.  A  SHORT  COURSE  IN   QUALITATIVE  ANALYSIS; 

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Chemistry  in  the  English  language."  etc.— Dublin  Med.  Journal. 

•*  MAGNETISM  AND  ELECTRICITY.   By  Wm.  Allen  Millei. 

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CHEMISTRY  — THEORETICAL,  PRACTICAL,  AITD 
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DRAWING  AND  PAINTING. 

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densed compilation  from  the  celebrated  jManuai  of  Bouvier, 
with  additional  matter  selected  from  the  labors  of  Merriwell, 
De  Montalbert,  and  other  disting-uished  Continental  writers 
on  the  art.  In  7  parts.  Adapted  for  a  Text-Book  in 
Academies  of  both  sexes,  as  well  as  for  self -instruction. 
Appended,  a  new  Explanatory  and  Critical  Vocabulary.  By 

an  American  Artist.    12mo,  cloth  $2  00 

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CONSTRUCTIVE  GEOMETRY  AND  INDUSTRIAL 
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I.  ELEMENTARY  WORKS. 

1.  ELEMENTARY  FREE-HAND  GEOMETRICAL  DRAWING. 

A  series  of  progressive  exercises  on  regular  lines  and  forms, 
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4.  ELEMENTARY  PROJECTION  DRAWING.    Revised  and  en- 

larged edition.  In  five  divisions.  This  and  the  last  volume 
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5.  ELEMENTARY  LINEAR  PERSPECTIVE  OF  FORMS  AND 

SHADOWS.  Part  I.— Primitive  Methods,  with  an  Introduc- 
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n.  HIGHi3R  WORKS. 

These  are  designed  principally  for  Schools  of  Engineering  and 
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■  provide  courses  of  study  adapted  to  the  preliminary  general 
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3.  HIGHER  LINEAR  PERSPECTIVE.    Distinguished  by  its  con- 

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chanical Establishments,  Artisans,  and  Inventors.  Containing 
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governors,  and  many  standard  and  novel  examples,  mostly  from 
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A  FEW  FROM  MANY  TESTIMONIALS. 

•*It  eeems  to  me  that  your  Works  only  need  a  thorough  examination  to  be  intro- 
duced and  permanently  used  in  all  the  Scientific  and  Engineering  Schools.'^ 
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**I  have  used  several  of  your  Elementary  Works,  and  believe  them  to  be  better 
'  adapted  to  the  purposes  of  instrnrtion  than  any  others  with  vi-hich  I  ara 
acquainted."— H.  F.  WALLING,  Prof,  of  Civil  and  Topographical  Engi- 
neering, Jjtfayette  College,  Eanton.  Pa. 

"Your  Works  appear  to  me  to  fill  a  very  impoi-tant  gap  in  the  literature  of  the 
subjects  treated.  Any  effort  to  draw  Ai-tisans,  etc.,  away  from  the  'rule  of 
Ihumb.'  and  give  them  an  insight  into  princii)lcH,  is  in  the  right  direction, 
and  meets  my  heartiest  approval.  This  is  the  distinguishing  feature  ot  your 
Elementary  Works." —Prof .  H.  L.  EUSTIS,  Lawrence  Scientific  School 
Cambridge,  Mass. 

••The  author  has  happily  divided  the  sabjocti  into  two  great  portions:  the  former 
"mbraoing  those  processes  and  iirobleius  i)roper  to  be  taught  to  all  students  in 
Institutions  of  Elementary  Instruction ;  the  latter,  those  suited  to  advanced 
students  preparing  for  technical  purposes.  The  Elementary  Books  ought  to 
be  uaed  in  all  High  Schools  and  Academies ;  the  Higher  ones  in  Rchools  of 
Technology."— WM.  W.  FOLWBLL,  President  of  UniveraUy  of  Minnetiotcu 


JOHN  WILEY  &  son's  LIST  OF  PUBLICATIONS. 


97 


DYEING,  Sec. 

MACFARLANE.  A  PRACTICAL  TREATISE  ON  DYEING  AND  CALICO- 
PRINTING.  Including  the  latest  Inventions  and  Improve- 
ments. With  an  Appendix,  <-omprising-  definitions  of  chemical 
terms,  with  tables  of  Weights,  Measures,  &c.  By  an  expe- 
rienced Dyer,  With  a  supplement,  containing  the  most 
recent  discoveries  in  color  chemistry.  By  Robert  Macfarlane. 
1  vol.  8vo  $5  00 

REIMANN.  A  TREATISE  ON  THE  MANUFACTURE  OF  ANILINR 

AND  ANILINE  COLORS.  By  IVL  Reimann.  To  which 
is  added  the  Report  on  the  ColdVing  Matters  derived  from 
Coal  Tar,  as  shown  at  the  French  Exhibition,  1867.  By  Dr. 
Hofmann.  Edited  by  Wm.  Crookes.  1  vol.  Svo,  cloth,  $2  50 
•'Dr.  Reiniann'a  portion  of  the  Ti'eatise,  written  in  concise  language,  is  profoundly 
practical,  giving  the  minutest  details  of  the  r.rocesses  for  obtaining  all  tha 
more  important  colors,  with  woudcuts  of  apparatus.  Taken  in  conjunction 
with  Hofinanu's  Report,  we  have  now  a  complete  liistory  of  Coal  Tar  Dyea, 
both  theoretical  and  practical." — Chemist  and  Druggist. 

ENGINEERING. 

AUSTIN.  A  PRACTICAL  TREATISE  ON  THE  PREPARATION, 

COMBINATION,  AND  APPLICATION  OF  CALCA- 
REOUS AND  HYDRAULIC  LIMES  AND  CEM  ENTS. 

To  which  is  added  many  useful  recipes  for  various  scientific, 
mercantile,  and  domestic  purposes.  By  James  G.  Austin. 
1  vol.  12mo  $2  0/0 

COLBURN  LOCOMOTIVE  ENGINEERING  AND  THE  MECHAN- 

ISM OF  RAILWAYS.  A  Treatise  on  the  Principles  and 
Construction  of  the  Locomotive  Engine,  Railway  Carriages, 
and  Railway  Plant,  with  examples.  Illustrated  by  Sixty-four 
large  engravings  ard  two  hundred  and  forty  woodcuts.  By 
Zerah  Colburn.    Complete,  20  parts,  $15.00;    or  2  vols. 

cloth  $16  00 

Or,  half  morccco,  gilt  top  $20  00 

KNIGHT.  THE  MECHANICIAN  AND  CONSTRUCTOR  FOR  BN: 
GINEEE.S.  Comprising  Forging,  Pla.ning,  Lining,  Slotting, 
Shaping,  Turning,  Screw-cutting,  &c.  Illustrated  with 
ninety-si.x  plates.  By  Cameron  ICnight.  1  vol.  4to,  half 
morocco  $15  00 

MAHAN.  AN  ELEMENTARY  COURSE  OF  CIVIL  ENGINEER- 
ING, for  the  use  of  the  Cadets  of  the  U.  S.  Military  Academy. 
By  D.  H.  Mahan.  1  vol.  Svo,  with  numerous  vv^oodcuta. 
New  edition.  Edited  by  Prof.  De  Volson  Wood.  Full 
cloth  $5  00 

«*  DESCRIPTIVE  GEOMETRY,  as  applied  to  the  Drawing  of 
Fortifications  and  Stone-Cutting.  For  the  use  of  the  Cadeta 
of  the  U.  S.  Military  Academy.  By  Prof.  D.  H.  Mahan. 
1  vol.  Svo.    Plates  $1  50' 

*•  INDUSTRIAL  DRAWING.  Comprising  the  Description  and 
Uses  of  Drawing  Instruments,  the  Construction  of  Plane 
Figures,  the  Projections  and  Sections  of  Geometrical  Solids, 
Architectural  Elements,  Mechanism,  and  Topographical 
Drawing.  With  remarks  on  the  method  of  Teaching  the 
subject.  For  the  use  of  Academies  and  Common  Schools. 
By  Prof.  D.  H.  Mahan.  1  vol.  Svo.  Twenty  steel  plates. 
Full  cloth  $3  00' 

«  A  TREATISE  ON  FIELD  FORTIFICATIONS.  Contain- 
ing instractions  on  the  Methods  of  Laying  Out,  Constructing, 
Defending,  and  Attacking  Entrenchments.  With  the  General 
Outlines,  also,  of  the  Arrangement,  the  Attack,  and  Defence 
of  Permanent  Fortifications.  By  Prof.  D.  H.  Mahan.  New 
edition,  revised  and  enlarged.  1  vol.  Svo,  full  cloth,  with 
plates  $3  50 

•  ELEMENTS  OF  PERMANENT  FORTIFICATIONS.  By 
Prof.  D.  H.  Mahan.  1  vol.  Svo,  with  numerous  large  plates. 
Cloth  $6  50 


98 


JOHN  WILEY  &  son's  LIST  OF  PUBLICATIONS. 


MAHAN.  ADVANCFD  GUARD,  OUT-POST  and  Detachment  Service 

of  Troops,  with  the  Essential  Principles  of  Strate^  and 
Grand  Tactics.  For  the  use  of  Officers  of  the  ]\Iilitla  nnd 
Volunteers.     By  Prof.  D.  H.  IMahan.    New  edition,  mth 

large  additions  and  12  plates.    1  vol.  18mo,  cloth  §1  50 

MAHAN  MECHANICAL    PRINCIPLPS    OF  ENGINTJERING 

&  MOSELY.        AND  ARCHITECTURE.   By  Henry  Mosely,  M.  A.,  F.R.S. 

From  last  London  edition,  with  considerable  additions,  by 
Prof.  D.  H.  Mahan,  LL.D.,  of  the  U.  S.  Military  Academy. 
1  vol.  8vo,  700  pages.    With  numerous  cuts.    Cloth. .  ..$5  00 
MAHAN  HYDRAULIC  MOTORS.    Translated  from  the  French  Cours 

&  BR  ESSE.        de  Mecanique,  appliqure  par  M.  Bresse.     By  Lieut.  F.  A. 

Mahan,  and  revised  by  Prof.  D.  II.  Mahan,     1  vol.  8vo, 

plates  |2  50 

WOOD,  A  TREATISE   ON   THE   RESISTANCE   OF  MATE- 

RIALS, and  an  Appendix  on  the  Preservation  of  Timber, 
By  De  Volson  Wood,  Professor  of  Engineering,  University  of 

Michigan.    1  vol.  8vo,  c'oth  ^3  50 

A  TREATISE  ON  BRIDGES.  Designed  as  a  Text-book  and 
for  Practical  Use.  By  De  Volson  Wood.  1  vol.  8vo,  nume- 
rous illustrations,  cloth  $3  00 

CREEK. 

BACSTER.  GREEK  TESTAMENT,   ETC.     The  Critical  Greek  and 

English  iS^ew  Testament  in  Parallel  Columns,  consisting  of 
the  Creek  Text  of  Scholz,  readings  of  Criesbach,  etc.,  etc. 

1  vol.  18mo,  half  morocco  $8  00 

**  ■   do.  Full  morocco,  gilt  edges   4  50 

"   With  Lexicon,  by  T  S.  Green.    Half-bound   4  50 

"    do.    Full  morocco,  gilt  edges   6  00 

**    do.    With  C-  ncordance  and  Lexicon.    Halfmor.,  6  00 

**    do.    Limp  morocco   7  50 

"  THE  ANALYTICAL  GREEK  LEXICON  TO  THE  NEW 

TESTAMENT.  In  which,  by  an  alphabetical  arrai  gement, 
is  foTind  every  word  in  the  Greek  text  in'  every  form  in  i.chich 
it  appears — that  is  to  say,  every  occurrent  person,  number, 
tense  or  mood  of  verbs,  every  case  and  number  of  nouns,  pro- 
noims,  &c. ,  is  plnced  in  its  alphabetical  order,  fu]!}^  explained 
by  a  careful  grammatical  analysis  and  referred  to  its  root,  so 
that  no  uncertainty  as  to  the  grammatical  stractnre  of  any 
word  can  perplex  the  beginner,  but,  assured  of  the.  j^recise 
grammatical  force  of  any  word  he  maj'  desire  to  ii^.terpret.  he 
is  able  immediately  to  apply  his  knowledge  of  the  English 
meaning  of  the  root  with  accuracy  and  satisfaction.    1  vol. 

small  4to,  half  bound  $6  50 

<»  GREEK-ENGLISH  LEXICON  TO  TESTAMENT.  By 
T.  S.  Green.    Half  morocco   $1  50 

HEBREW. 

CREEN.  A  GRAMMAR  OF  THE  HEBREW  LANGUAGE.  With 

copious  Appendixes.  By  W.  H.  Green,  D.D.,  Professor  in 
Princetnn  Theological  Seminary.    1  vol.  8vo,  cl'  th  50 

"  AN  ELEMENTARY  HEBREW  GRAMMAR.  With 
Tables,  Reading  Exercises,  and  Vocabulary.  By  Prof.  W.  II. 
Green,  D.D.    1vol.  12mo,  cloth   $1  50 

"  HEBREW  CHRESTOMATHY;  or.  Lessons  in  Readii.^-  ;.nd 

Writing  Hebrew.    By  Prof.  W.  H.  Green,  D.D.    1  vol.  Svo, 

cloth  .>j;3  00 

LETTERIS.  A  NEW  AND  BEAUTIFUL  EDITION  OF  THE  HE- 

BREW BIBLE.   Re\nsed  and  carefully  examined  by  Myor 

Le\'i  Letteris.    1  vol.  8vo,  wilh  key,  marble  edg'-s  $^2  50 

"This  edition  has  a  larpe  and  mnch  more  legible  type  than  the  known  onevohuna 
editions,  and  the  piinb  is  excellent,  while  the  name  of  Letteris  ia  a  snfficicnt 
guarantee  for  correctnees."  -Rev.  Dr.  J.  M.  WISE,  Editor  qf  the  ISBAixrnt. 


JOHN  WILEY  &  son's  LIST  OF  PUBLIC A.TIONSr 


99 


BACSTER'S        BAGSTER'S  COMPLETE   EDITION  OF  GESENIUS' 
CESENIUS.        HEBREW  AND  OHALDEH  LEXICON.     In  la^ge 
clear,  and  perfect  type.    Translated  and  edited  with  addi- 
tions and  corrections,  by  S,  P.  Tregelles,  LL.D. 
In  this  edition  preat  care  has  been  taken  to  giiard  the  student  from  Neologian 

tendencies  by  suitable  remarks  whenever  needed. 
"The  careful  revisal  to  which  the  Lexicon  has  been  subjected  by  a  faithful  and 
Orthodox  translator  exceedingly  enhances  the  practical  value  of  this  edition." 
— Edinburrih  Eccleaimtical  Journal. 

SmaU  4to,  half  bound  $7  50 

BACSTER'S        NEW  POCKET  HEBREW  AND  ENGLISH  LEXICON. 

The  arrangement  of  this  Manual  Lexicon  combiaes  two 
things — the  etymological  order  of  roots  and  the  alphabetical 
order  of  words.  This  arrangement  tends  to  lead  the  learner 
onward;  for,  as  he  becomes  more  at  home  with  roots  and 
derivatives,  he  learns  to  turn  at  once  to  the  root,  without  lirst 
searching  for  the  particular  word  in  its  alphabetic  order.  1 

vol.  18mo,  cloth  $2  00 

*' This  is  the  most  beautiful,  and  at  the  same  time  the  most  correct  and  pei  feet 
Manual  Hebrew  Lexicon  we  have  ever  used." — Eclectic  Eeview. 


IRON,  METALLURGY,  &c. 

BODEMANN.       A  TREATISE  ON  THE  ASSAYING  OF  LEAD,  SILVER, 

COPPER,  GOLD,  AND  MERCURY.  By  Bodemann  & 
Kerl.    Translated  by  W.  A.  Goodyear.    1  vol.  12mo,  $2  50 

CROOKES.  A  PRACTICAL  TREATISE  ON  METALLURGY.  Adap- 

ted from  the  last  German  edition  of  Prof.  Kerl's  Metallurgy. 
By  William  Crookes  and  Ernst  Rohrig.    In  three  vols,  thick 

8vo.    Price  $30  00 

Separately.  Vol.  1.  Lead,  Silver,  Zinc,  Cadmium,  Tin.  Mer- 
cury, Bismuth,  Antimony,  Nickel,  Arsenic,  Gold,  Platinum, 

and  Sulphur  ".  $10  00 

Vol.  2.  Copper  and  Iron   10  00 

Vol.  3.  Steel,  Fuel,  and  Supplement   10  00 

FAIRBAIRN.         CAST  AND  WROUGHT  IRON  FOR  BUILDING.  By 

Wm.  Fairbaim.    8vo,  cloth  $2  00 

FRENCH.  HISTORY  OF  IRON  TRADE,  FROM  1621  TO  1857.  By 

B.  F.  French.    8vo,  cloth  $2  00 

KIRKWOOD  COLLECTION  OF  REPORTS  (CONDENSED)  AND 
OPINIONS  OF  CHEMISTS  IN  REGARD  TO  THE 
USE  OP  LEAD  PIPE  FOR  SERVICE  PIPE,  in  the 
Distribution  of  Water  for  the  Supply  of  Cities.  By  I.  P. 
Kirkwood,  C.E.    8 vo,  cloth  $150 

LESLEY.  THE  IRON  MANUFACTURER'S  GUIDE  TO  THE 
FURNACES,  FORGES,  AND  ROLLING-MILLS  OF 
THE  UNITED  STATES.  By  J.  P.  Lesley.  With  tnaps 
and  plates.    1  vol.  8vo,  cloth  $S  00 


FITZGERALD. 


HOLLY. 


KNIGHT. 


MACHINISTS-MECHANICS. 

TH.E  BOSTON  MACHINIST.  A  complete  School  for  the 
Apprentice  and  Advanced  Machinist.  By  W.  Fitzgerald.  1 
vol.  ISmo,  cloth  $0  75 

SAV/  FILING.  The  Art  or  Saw  Filing  Scientifically  Treated 
and  Explained.  With  Directions  for  putting  in  order  all  kinds 
of  Saws,  from  a  Jeweller's  Saw  to  a  Steam  Saw-mill.  Illus- 
trated by  forty -four  engravings.  Third  edition.  By  H.  W. 
Holly.    1  vol.  ■  1 8mo,  cloth  $0  75 

THi;  MiaOHAMISM  AND  ENGIN!?BR  INSTRUCTOR. 
Comprising  Forging,  Planing,  Lining,  Slotting,  Shaping. 
Turning,  Screw-Cutting,  etc.,  etc.  By  Cameron  Knight.  1 
yoL  4to,  half  morocco  $15  OC 


100 


JOHN  WILEY  &  son's  LIST  OF  PUBLICATIONS. 


TURNING,  &c.     LATHU,  THE,  AND  ITS  USES,  ETC.;  or^  Instruction  in 

the  Art  of  Tiirrdng  Wood  and  Metal.  Including  a  descrip- 
tion of  the  most  modem  appliances  for  the  ornamentation  of 
plane  and  curved  surfaces,  with  a  description  also  of  an 
entirely  novel  form  of  Lathe  for  Eccentric  and  Rose  Engine 
Turning-,  a  Lathe  and  Turning  Machine  combined,  and  other 
valuable  matter  relating  to  the  Art.  1  vol.  8vo,  copiously 
illustrated.     Including  Supplement.    8vo,  cloth. ....  .$7  00 

"The  most  complete  work  on  the  subject  ever  published." — American  Artisan. 

"  Here  is  an  invaluable  book  to  the  practical  workman  and  amateur." — London 
Weekly  Times. 

TURNING,  &c.     SUPPLEMENT  AND  INDEX  TO  LATHE  AND  ITS 
USES.    Large  type.    Paper,  8vo  $0  90 

WILLIS.  PRINCIPLES  OF  MECHANISM.    Designed  for  the  use  of 

Students  in  the  Universities  and  for  Engineering  Students 
generally.  By  R  -bert  Willis,  M.D  ,  F.R.S.,  President  of  the 
British  Association  for  the  Advancement  of  Science,  &c.,  &c. 

Second  edition,  enlarged.    1  vol.  Svo,  cloth  $7  50 

It  ought  to  be  in  every  large  Machine  Workshop  Office,  in  every  School  of 
Mechanical  Engineering  at  least,  and  in  the  hands  of  every  Profeswr  of 
Mechanics,  &c.— Prof.  S.  EDWARD  WAEREN. 


BOOTH. 


CELDARD. 


BULL 


FRANCKE 


GREEN. 


MANUFACTURES. 

NEW  AND  COMPLETE  CLOCK  AND  WATCH 
MAKEHS'  MANUAL.  Comprising  descriptions  of  the 
various  gearings,  escapements,  and  Compensations  now  in 
use  in  French,  Swiss,  and  English  clocks  and  watches.  Patents, 
Tools,  etc. ,  with  directions  for  cleaning  and  repairing.  With 
numerous  engravings.  Compiled  from  the  French,  with  an 
Appendix  containing  a  History  of  Clock  and  Watch  Making  in 
America.  By  Mary  L.  Booth.  -  With  numerous  plates.  1 
vol.  12mo,  cloth  $2  00 

HANDBOOK   ON   COTTON  MANUFACTURE;    or,  A 

Guide  to  Machine-Building,   Spinning,   and  Weaving. 

With  practical  examples,  all  needful  calculations,  and  many 
useful  and  important  tables.  The  whole  intended  to  be  a 
complete  yet  compact  authority  for  the  manufacture  of 
cotton.  By  James  Geldard.  With  steel  engravings.  1  vol. 
12mo,  cloth  $2  50 

MEDICAL,  &c. 

HINTS  TO  MOTHERS  FOR  THE  MANAGEMENT  OF 
HEALTH  DURING  THE  PERIOD  OF  PREG- 
NANCY, AND  IN  THE  LYING-IN  ROOM.  With  an 
exposure  of  popular  errors  in  connection  with  those  subjects. 
By  Thomas  Bull,  M.D.    1  vol.  12mo,  cloth  $1  00 

OUTLINES  OF  A  NEW  THEORY  OF  DISEASE,  appHed 
to  Hydropathy,  showing  that  water  is  the  only  true  remedy. 
With  observations  on  the  errors  committed  in  the  practice  oi 
Hydropathy,  notes  on  the  cure  of  cholera  by  cold  water,  and 
a  critique  on  Priessnitz's  mode  of  treatment.  Intended  foi 
popular  use.  By  the  late  H.  Francke.  Translated  from  the 
German  by  Robert,  Blakie,  M.D.    1  vol.  12mo,  cloth. .  .$1  50 

A  TR."5jATISE  on  DISEASES  OF  THE  AIR  PASSAGES. 

Comprising  an  inquiry  into  the  History,  Pathology,  Causes, 
and  Treatment  of  those  Affections  of  the  Throat  caUed  Bron- 
chitis, Chronic  Larj'ngitis,  Clergyman's  Sore  Throat,  etc.,  eta 
By  Horace  Green,  M.D.  Fourth  edition,  revised  and  enlarged, 
1  "vol.  Svo,  cloth  §3  06 

A  P  -  CTIC.\L  TREATISE  ON  PULMONARY  TUBER- 
CULOSIS, embracing  its  History,  Pathology,  and  Treat- 
ment. By  Horace  Green,  MD.  Colored  platoa  1  vol.  Svo, 
cloth  $5  0(1 


JOHN  WILEY  &  son's  LIST  OF  PUBLICATIONS. 


101 


BRUSH. 
DANA. 


CREEM.  OBSERVATIONS  ON  THB  PATHOLOGY  OF  CROUP 

With  Remarks  on  its  Treatment  by  Topical  Medications,  ijy 

Horace  Green,  M.D.    1  vol.  8vo,  cloth  $1  2 J 

"  ON  THE  SURGICAL  TREATMENT  OP  POLYPI  OF 
THE  LARYNX,  AND  OGDEMA  OF  TKIi  GLOTTIS. 
By  Horace  Green,  M.D.    1  vol.  8vo  $1  25 

"  FAVORITE  PRESCRIPTIONS  OF  LIVING  PRACTI- 
TIONERS. With  a  Toxicol og-ical  Table,  exhibiting  the 
Symptoms  of  Poisoning-,  the  Antidotes  for  each  Poison,  and 
the  Test  proper  for  their  detection.  By  Horace  Green.  1 
vol.  8vo,  cloth  |2  50 

TILT.  ON  THE  PRESERVATION  OF  THE  HEALTH  OF 
WOMEN  AT  THE  CRITICAL  PERIODS  OF  LIFE. 
By  E.  G.  Tilt,  M.D.    1  vol.  ISmo,  cloth  $0  50 

VON  DUBEN.  GUSTAF  VON  DUBEN'S  TREATISE  ON  MICRO- 
SCOPICAL DLIGNOSIS.  With  71  engravings.  Trans- 
lated, with  additions,  by  Prof.  Louis  Bauer,  M.  D.  1  vol.  8vo, 
cloth  $1  00 

MINERALOGY. 

ON  BLOW-PIPE  ANALYSIS.    By  Prof.  Geo.  J.  Bmsk.  (In 

preparation. ) 

DESCRIPTIVE  MINERALOGY.  Comprising  the  most  re- 
cent Discoveries.  Fifth  edition.  Almost  entirely  re- written 
and  greatly  enlarged.  Containing  nearly  900  pages  8vo,  and 
upwards  of   600  wood  engravings.     By  Prof.  J.  Dana. 

Cloth  $10  00 

"We  have  nsed  a  good  many  works  on  Mineralogy,  but  have  met  with  none  that 
begin  to  compare  with  this  in  fulness  of  plan,  detail,  and  execution," — 
American  Journal  of  Mining. 

APPENDIX  TO  DANA'S  MINERALOGY,  bringing  the 
work  down  to  1872.    By  Prof.  G.  J.  Brush.    3vo  $0  50 

DETERMINATIVE  MINERALOGY.  1  vol.  (In  prepa- 
ration.) ^ 

A  TEXT-BOOK  OF  MINERALOGY.  1  vol.  (In  prepa- 
ration. ) 

MISCELLANEOUS. 

THE  NEW  TALE  OF  A  TUB.  An  adventure  in  verse.  By 
F.  W.  N.  Bailey.    With  illustrations.    1  vol.  8vo  $0  75 

ON  HEROES,  HERO-V/ORSHIP,  AND  THE  HEROIC  IN 
HISTORY.  Six  Lectures.  Keported,  with  emendations  and 
additions.    By  Thomas  Carlyle.    1  voL  12mo,  cloth. .  .|0  75 

CATLIN.  .   THE  BREATH  OF  LIFE;  or,  Mai-Respiration  and  its 

Effects  upon  the  Enjoyments  and  Life  of  Man.    By  Geo. 

Catlin.  With  numerous  wood  engravings.  1  vol.  Svo,  $0  75 
CHEEVER.  CAPITAL  PUNISHMENT.   A  Defence  of.    By  Rev.  George 

B.  Cheever,  D.D.    Cloth  $0  50 

"  HI.LL  DIFFICULTY,  and  other  Miscellanies.     By  Rev. 

George  B.  Cheever,  D.D.    1  voL  12mo,  cloth  $1  00 

*»  JOURNAL  OF  THE  PILGRIMS  AT  PLYMOUTH  ROCK. 

By  Geo.  B.  Cheever,  D.D.    1  voL  12mo,  cloth  $1  00 

*  WANDERINGS  OF  A  PILGRIM  IN  THE  ALPS.  By 

George  B.  Cheever,  D.D.    1  vol.  12mo,  cloth  $1  00 

**  WANDERINGS  OF  THE  RIVER  OF  THE  WATER  OF 

LIFE.    By  Rev.  Dr.  George  B.  Cheever.     1  vol.  12mo, 

cloth  $1  00 

CONYBEARE.      ON  INFIDELITY.    12mo,  cloth   1  00 

CHILD'S  BOOK   OF  FAVORITE  STORIES.     Large  colored  platea  4to, 

cloth.  $1  50 


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DANA. 


BAILEY. 
CARLYLE. 


102 


JOHN  WILEY  &  son's  LIST  OF  PUBLICATIONS. 


HEIGHWAY. 
KELLY. 


EDWARDS.         FREF  TOWN  LIPRARIEP.    The  Formation,  Mana^-ement 

and  History  in  Britain,  France,  Germanj^,  and  America. 
Tog-ether  with  brief  notices  of  book-collec;tors,  and  ol  the 
respective  places  of  deposit  of  their  surviving  collections. 
By  Edward  Edwards.    1  vol.  thick  8vo  ^4  00 

GREEN.  THE  PENTATEUCH  VINDICATED  FROM  THE  AS. 

PERSIONS  OF  BISHOP  COLENSO.  By  Wm.  Henry 
Green,  Prof.  Theological  >eniinary,  Princeton,  X.  J.  1  voL 
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